DistanceVectorRouting_5

8/10/2019 DistanceVectorRouting_5

http://slidepdf.com/reader/full/distancevectorrouting5 1/122

Distance Vector Routing and RIP

Justinian Anatory



Department of Computer and systems Engineering 4

Distance Vector Routing Updates

• RIP – Hop Count• IGRP and EIGRP – Bandwidth, Delay, Reliability, Load

• Cisco’s OSPF – Bandwidth

• IS-IS – Cost

• BGP – Number of AS or policy

No! MTU is never

used as a routingmetric. Some

documentation is

incorrect on this

item.




FAQs

FAQs – Network Discovery

Q: How often does initial network discovery happen?

A: Only when the network comes first comes up.

Q: Do routers share routing table information after network discovery?

A: Yes, distance-vector routing protocols share their entire routing tables

periodically (with or without split horizon enabled). Distance vector routing

protocols on Cisco routers by default use split horizon with poison reverse(discussed in the next section). Depending upon the distance-vector routing

protocol, the frequency of the updates will happen for RIP every 30 seconds,

IPX RIP every 60 seconds, and IGRP every 90 seconds.

Q: What happens when there is a change in the topology, link goes down, newnetwork is added, new router, is added, etc.?

A: Let’s take a look.




Triggered Updates - Extra

Triggered Updates

• Routers do not have to wait for the periodic update to hear aboutchanges in the network topology.

• Improvements to the distance-vector algorithm is typically made indistance-vector routing protocols, like RIP, to include triggeredupdates.

• Even with triggered updates, large distance vector networks can suffer

from long convergence times in some situations.




Triggered Updates

• Triggered updates are sent whenever a router sees a topology changeor a change in routing information (from another router).

• The router does not have to wait for the period timer, but can sendthem immediately.

• Triggered updates do not need to include the entire routing table butonly the modified route(s).

• Triggered updates must still be sent to adjacent routers, from router to

router, like other routing updates.




Triggered Updates

• Most distance-vector routing protocols limit the frequency of triggered

updates so that a flapping link does not put an unnecessary load on

the network. (RIP: random 1 to 5 seconds)

• Typically, triggered updates can be “triggered” by: – Interface transition to the up or down state

– A route has entered or exited an unreachable (down) state (later)

– A new route is installed in the routing table




Routing Loop Issues

Routing Loops

• Distance vector routing protocols are simple in their implementation and configuration,

but this comes at a price.

• Pure distance vector routing protocols suffer from possible routing loops.

• Routing loops can cause major network problems, from packets getting lost (blackholed)in your network, to bringing down your entire network.

• Several remedies to have been added to distance-vector algorithms to help prevent

routing loops including:

– Split horizon

– Hold-down timers – Defining a maximum metric




Routing Loop Issues

What can cause routing loops?

• Routing loops can occur when there are:

– Incorrect or inconsistent routing updates due to slow convergence after a

topology change. (Example coming up next.) – Incorrect or incomplete routing information (see presentation on Discard

Routes)

– Static routes incorrectly configured with an intermediate address whichdoes not become resolved in the routing table. (see presentation on Static

Routes – Additional Information)




Routing Loop Issues

Routing Loop Example

• Assume for the remainder of this example that Router C’s preferred path to

network 1 is by way of Router B.• Router C’s routing table has a distance of 3 to network 1 via Router B.




Routing Loop Issues

Network 1 Fails

• Router E sends an update to Router A.

• Router A stops routing packets to network 1.

• But Routers B, C, and D continue to do so because they have not yet beeninformed about the failure.

• Router A sends out its update.

• Routers B and D stop routing to network1, (via Router A).

• However, Router C is still not updated.

•To router C, network 1 is still reachable via router B.




Routing Loop Issues

Router C sends a periodic update to Router D

• Router C sends a periodic update to Router D indicating a path to network 1(by way) of via Router B. (4 hops).

Router D’s Routing Table information for Network 1

• Current path to Network 1 = Unreachable (down)

• Information from Router C: Network 1 : 4 hops by way of Router C

• Normally, RouterD ignores this routing information because it usually has abetter route, 2 hops, via Router A, but this route is now down.

• Router D changes its routing table to reflect this (good) better , but incorrect information, Network 1 by way of Router C (4 hops)

• Router D propagates the information to Router A.




Routing Loop Issues

Routers A changes its routing table

• Router A adds new route to its routing table, get to Network 1 by way of RouterD (5 hops).

• Propagates the information to Routers B and E.

Router B (and Router E) change their routing tables

• Router B now believes it can get to Network 1 by way of Router A (6 hops).

Wow! I was about to tell Router C that Network 1 was down via Router B, butnow I have new information!

•

Propagates the incorrect information to Router C.




Routing Loop Issues

Router C changes its routing table

• Router C still believes it can get to Network 1 by way of Router B (7 hops).

Of course now it believes it is 7 hops instead of 3.

• Propagates the newer but still incorrect information to Router D.

Here we go again!

• Data packets destined for Network 1 get caught in a routing loop, from Routers A to D to C to B to A to D etc.

• As routing updates continue between the routers, the hop count gets greater –

to infinity? (Not quite – we will see in a moment.)




Defining a Maximum

Problem: Count to infinity

Solution: Defining a Maximum

• Distance vector routing algorithms are self-correcting, but a routing loop

problem can require a count to infinity.

• To avoid this prolonged problem, distance vector protocols define infinity as aspecific maximum number.

• This number refers to a routing metric which may simply be the hop count.

• When the metric value exceeds the maximum value, and as each router

receives this maximum metric, the network is then considered unreachable.




Why only a 15 hop count limit?

Question: Why does RIP use a hop count as the route metric, and why is its

maximum value limited to 15?

Answer : “When RIP was designed and implemented, dynamic routing protocols

were not widely used. Instead, networks relied mostly on static routing. RIP,

even with its hop-count-metric – which seems very poor to us today – was

quite a big improvement. Counting intermediate routes is the simplest method

to measure the quality of routes. Setting the infinity value for the metric isalways a problem of choosing between wider networks and faster convergence

when the protocol starts counting. When RIP was invented, it seemed unlikely

to have a network with the maximum diameter more more than 15 routers, so

16 was chosen as the infinity value.”




Split Horizon

• “The effect of split horizon is that a router will send out different routing

messages on different interfaces. In effect a router never sends out

information on an interface that it learned from that interface.”




Split Horizon

• “Split-horizon attempts to avoid this situation. If a routing update aboutNetwork 1 arrives from Router A, Router B or Router D cannot send

information about Network 1 back to Router A. Split-horizon thusreduces incorrect routing information and reduces routing overhead.”

• Initially, this is true, but the loop is a result of Router C sending out theupdates, because it has not converged.




Split Horizon Rule – Avoiding Routing Loops

• Routers RTA and RTB have their initial routing tables and are ready to

exchange routing information via a distance-vector routing protocol

like RIP.Split Horizon disabled

• If split horizon were disabled the routing updates would include all of

the networks in their routing tables including their directly connected

networks and any networks learned from any interface.

RTA RTB10.1.1.0/24

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

Simple Split Horizon

Initial

routingtables




Split Horizon Disabled

• After the initial exchange of updates everything in the routing tableslook fine.

• Because split horizon disabled, the 10.1.2.0/24 network is sent by bothrouters, but neither router includes the other’s route to 10.1.2.0/24 (1hop) in the routing table, because it has a current route with a bettermetric of 0.

Initialrouting

tables

Newrouting

tables

10.1.2.0/24network is

included because

split horizon has

been disabled

RTA RTB10.1.1.0/24

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

Routing Update

Next-hopNet. Hops Address

10.1.1.0/24 1 10.1.1.1

10.1.2.0/24 1 10.1.1.1

Routing Update

Next-hop

Net. Hops Address

10.1.2.0/24 1 10.1.2.2

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

10.1.1.0/24 1 10.1.2.1




Split Horizon Disabled – 10.1.3.0/24 down

• Note: Routing tables are not sent at the exactly same time. We willlearn about this in Routing Protocols, that this is done on purpose to

avoid collisions on broadcast networks like Ethernet.

• Here, the 10.1.3.0/24 network fails, and before RTB sends out its

routing update, RTB receives a routing update from RTA.

RTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Update


10.1.1.0/24 1 10.1.1.1

10.1.2.0/24 1 10.1.1.1

10.1.3.0/24 2 10.1.1.1

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

10.1.1.0/24 1 10.1.2.1

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 2 10.1.2.1

10.1.1.0/24 1 10.1.2.1

X

Previousrouting

tables

Networks in redwere included

because split

horizon has been

disabled

Newrouting

tables





• RTB notices that it has a route to 10.1.3.0/24 via RTA. Even though it is 2hops it is certainly better than its current situation of “unreachable” so itaccepts this better, but incorrect information from RTA.

• RTB now forwards all packets destined for 10.1.3.0/24 to RTA at 10.1.2.1.

• RTA receives these packets and forwards them to RTB at 10.1.2.2.

• RTB forwards them back to RTA at 10.1.2.1.• And so on! The packets get blackholed in this routing loop.

RTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Update


10.1.1.0/24 1 10.1.1.1

10.1.2.0/24 1 10.1.1.1

10.1.3.0/24 2 10.1.1.1

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

10.1.1.0/24 1 10.1.2.1

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 2 10.1.2.1

10.1.1.0/24 1 10.1.2.1

Previousrouting

tables

Networks in red

were included

because split

horizon has been

disabled

New

routing

tables





• Meanwhile, its RTB’s turn to send its routing update.• RTB increments the hop count to 10.1.3.0/24 to 3 hops and sends it to

RTA.

• When RTA sends out its next routing table it will increment the hop

count to 10.1.3.0/24 to 4 hops and sends it to RTB.• And on and on, until “infinity” which in RIP is 16 hops.

Previousrouting

tables

Networks in red

were included

because split

horizon has been

disabled

New

routing

tables

RTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 3 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 2 10.1.2.1

10.1.1.0/24 1 10.1.2.1

X

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 2 10.1.2.1

10.1.1.0/24 1 10.1.2.1

Routing Update


10.1.2.0/24 1 10.1.2.2

10.1.3.0/24 3 10.1.2.2

10.1.1.0/24 2 10.1.2.2




Split Horizon Disabled

• Once both routers have 16 hops for 10.1.3.0/24, they will both markthis network as unreachable and discontinue forwarding, drop, packetsto this network.

• This temporary routing loop can be easily avoided by enabling splithorizon on the serial 0 interfaces.

• Split horizon rule states that router never sends out information on aninterface that it learned from that interface

• Let’s see!

RTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

X

Routing Table

Net. Hops Ex-Int10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 16 10.1.2.2

Routing Table

Net. Hops Ex-Int10.1.2.0/24 0 s0

10.1.3.0/24 16 10.1.2.1

10.1.1.0/24 1 10.1.2.1

Simple Split Horizon




RTA RTB10.1.1.0/24

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

Routing Update


10.1.1.0/24 1 10.1.1.1

Routing Update


10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

10.1.1.0/24 1 10.1.2.1

Routing Update

Next-hop

Net. Hops Address

10.1.1.0/24 1 10.1.1.1

Routing Update

Next-hop

Net. Hops Address

10.1.3.0/24 1 10.1.2.2

Previousrouting

tables

Newrouting

tables

Split

HorizonEnabled




Split Horizon Enabled

• As you can see, with split horizon enabled, RTA does not send RTB (out s0)information about 10.1.3.0/24 because it learned it from RTB (same s0), andRTB does not send RTA (out s0) information about 10.1.1.0/24 to RTAbecause it learned it from RTA (same s0). (This also includes the commonnetwork between them.

RTA RTB10.1.1.0/24

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

Routing Update

Next-hop

Net. Hops Address

10.1.1.0/24 1 10.1.1.1

Routing Update

Next-hop

Net. Hops Address

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

10.1.1.0/24 1 10.1.2.1

Routing Update

Next-hop

Net. Hops Address

10.1.1.0/24 1 10.1.1.1

Routing Update

Next-hop

Net. Hops Address

10.1.3.0/24 1 10.1.2.2

Previousrouting

tables

New

routing

tables




Split Horizon Enabled – 10.1.3.0/24 down

• RTB notices 10.1.3.0/24 is down and puts this route into hold-down state inits routing table. (hold-down coming next)

• RTB immediately sends out a triggered update for only this route (if there wereothers in the routing table) with a metric of infinity, 16.

• RTA receives the triggered update and puts the route for 10.1.3.0/24 intohold-down state.

Previous

routing

tables

Newrouting

tables

RTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 (down) e0

10.1.1.0/24 1 10.1.2.1

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 (down) 10.1.2.2

X

Routing Update Next-hop

Net. Hops Address

10.1.3.0/24 16 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 (down) e0

10.1.1.0/24 1 10.1.2.1




Split Horizon Enabled – 10.1.3.0/24 down

• Notice that RTA never sends RTB a routing update for 10.1.3.0/24,

because split horizon is enabled on these interfaces.

Previous

routing

tables

Newrouting

tables

RTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 (down) e0

10.1.1.0/24 1 10.1.2.1

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 (down) 10.1.2.2

X

Routing Update Next-hop

Net. Hops Address

10.1.3.0/24 16 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 (down) e0

10.1.1.0/24 1 10.1.2.1




Split Horizon with Poison Reverse

• Many vendor implementations of distance vector routing protocols like Cisco’s RIP and

IGRP apply a special kind of split horizon, called split horizon with poison reverse.

• “Split horizon with poison reverse means that, instead of not advertising routes to the

source, routes are advertised back to the source with a metric of 16, which will make thesource router ignore the route. It is perceived that explicitly telling a router to ignore a

route is better than not telling it about the route in the first place.” (Lewis, Cisco TCP/IP

Routing)

• One drawback is that routing update packet sizes will be increased when using Poison

Reverse, since they now include these routes.

Split Horizon with Poison ReverseRTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

10.1.1.0/24 1 10.1.2.1

Routing Update

Next-hop

Net. Hops Address

10.1.1.0/24 1 10.1.1.1

10.1.2.0/24 16 10.1.2.1

10.1.3.0/24 16 10.1.2.1

Routing Update

Next-hop

Net. Hops Address

10.1.3.0/24 1 10.1.2.2

10.1.2.0/24 16 10.1.2.2

10.1.1.0/24 16 10.1.2.2

“Poisoned”

routes in red.

Routing tables

remain the

same.




Split Horizon Enabled by Default

Split horizon with poison reverse is enabled by default for all interfaces except :

• Physical interfaces or multipoint sub-interfaces using Frame Relay or SMDSencapsulation

To disable split horizon on an interface:

Router(config-if)# no ip split-horizon

To enable split horizon on an interface:

Router(config-if)# ip split-horizon

RTA RTB10.1.1.0/24 10.1.2.0/24 10.1.3.0/24

s0 s0e0 e0

.1 .1.1 .2

Routing Table

Net. Hops Ex-Int

10.1.1.0/24 0 e0

10.1.2.0/24 0 s0

10.1.3.0/24 1 10.1.2.2

Routing Table

Net. Hops Ex-Int

10.1.2.0/24 0 s0

10.1.3.0/24 0 e0

10.1.1.0/24 1 10.1.2.1

Routing Update

Next-hop

Net. Hops Address

10.1.1.0/24 1 10.1.1.1

10.1.2.0/24 16 10.1.2.1

10.1.3.0/24 16 10.1.2.1

Routing Update

Next-hop

Net. Hops Address

10.1.3.0/24 1 10.1.2.2

10.1.2.0/24 16 10.1.2.2

10.1.1.0/24 16 10.1.2.2

“Poisoned”

routes in red.

Split Horizon with Poison Reverse




Route poisoning

• When route poisoning is used with triggered updates it will speed up

convergence time because neighboring routers do not have to wait 30

seconds before advertising the poisoned route.

Pre enting ro ting loops ith holddo n




Preventing routing loops with holddown

timers

• The main function of holddown timers is to prevent the distance vectorrouting protocol from establishing routing loops during periods of networktransition (topology changes).

• “The rule: Once a route is marked unreachable, it must stay in this state for aperiod of time assumed sufficient for all routers to receive new informationabout the unreachable network. In essence, we instruct the routers to let therumors calm down and then to pick up the truth.” (Zinin, Cisco IP Routing)

• The amount of time a router remains in “this state” is determined by theholddown timer .






timers

Curriculum

• A count to infinity problem can be avoided by using holddown timers.

• When a router receives an update from a neighbor indicating that a

previously accessible network is now inaccessible, the router marksthe route as inaccessible and starts a hold-down timer.






timers

Same Route from same neighbor: Network is back up (Correct News)

•If at any time before the hold-down timer expires an update is received

from the same neighbor indicating that the network is again accessible,

the router marks the network as accessible and removes the hold-

down timer.






timers

Better Route from different neighbor (Correct News)

•If at any time before the hold-down timer expires an update arrives

from a different neighboring router with a better metric than originally

recorded for the network, the router marks the network as accessible

and removes the hold-down timer.






timers

Poorer Route from a different neighbor. (Incorrect News)

• If at any time before the hold-down timer expires an update arrives from a

different neighboring router with a poorer metric than originally recorded for thenetwork the update is ignored and the hold-down timer continues.

• Ignoring an update with a poorer metric when a hold-down is in effect allows

more time for the knowledge of a disruptive change to propagate through the

entire network.






timers

Additional Information on Holddown Timers

Flapping routes

• Holddown timers not only help prevent routing loops during transient periodsbut also help network stability by dampening unstable, flapping routes (routes

which continuously go up and down).

Holddown Time

• As we will see with both RIP and IGRP, the amount of time the router remainsin the holddown state can be modified (with caution!), even set to 0.

• We will look at this later in the presentations on RIP and IGRP.






timers

Additional Information on Holddown Timers

Packet forwarding

• Even though routing tables remain constant and routers do not acceptpotentially bad updates, an interesting question is whether or not routers

should continue use the existing routes that are in holddown state forforwarding packets?

• “In practice, routes in the holddown state are used for packet forwarding. Inthe worst case, packets are forwarded toward the router that was previouslyconnected to the destination network, which drops them. In the best case, theyare forwarded along a potentially suboptimal but valid path.” (Zinin, Cisco IPRouting)

Avoiding routing loops with triggered




Avoiding routing loops with triggered

updates

• Triggered update is sent immediately in response to some change inthe routing table.

• The router that detects a topology change immediately sends an updatemessage to adjacent routers that, in turn, generate triggered updates

notifying their adjacent neighbors of the change.• When a route fails, an update is sent immediately rather than waiting on

the update timer to expire.

• Triggered updates, used in conjunction with route poisoning, ensure

that all routers know of failed routes before any holddown timers canexpire.

IP’ TTL Ti T Li fi ld




Let’s look at a related item in IP, the TTL field.

IP Header

0 15 16 31

4-bit

Version

4-bit

HeaderLength

8-bit Type Of

Service(TOS)

16-bit Total Length (in bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

IP’s TTL – Time To Live field





• When a packet is first generated a value is entered into the TTL field.

• Originally, the TTL field was the number of seconds, but this was difficult to implement

and rarely supported.

• Now, the TTL is now set to a specific value which is then decremented by each router.

IP Header

0 15 16 31

4-bit

Version

4-bit

HeaderLength

8-bit Type Of

Service(TOS)


16-bit Identification3-bitFlags 13-bit Fragment Offset

8 bit Time To Live

TTL




Options (if any)

Data






Decrement by 1, if 0 drop the

packet.

IP Header

0 15 16 31

4-bitVersion

4-bitHeader

Length

8-bit Type OfService

(TOS)


16-bit Identification

3-bit

Flags 13-bit Fragment Offset

8 bit Time To LiveTTL




Options (if any)

Data


• If the router decrements the TTL field to 0, it will then drop the packet (unless the packet

is destined specifically for the router, I.e. ping, telnet, etc.).• Common operating system TTL values are:

– UNIX: 255

– Linux: 64 or 255 depending upon vendor and version

– Microsoft Windows 95: 32

– Other Microsoft Windows operating systems: 128

http://www.switch.ch/docs/ttl_default.html




TTL Overview - Disclaimer:

The following list is a best effort overview of some widely used TCP/IP stacks. Theinformation was provided by vendors and many helpful system administrators. We would

like to thank all these contributors for their precious help ! SWITCH cannot, however,take any responsibility that the provided information is correct. Furthermore, SWITCHcannot be made liable for any damage that may arise by the use of this information.

+--------------------+-------+---------+---------+

| OS Version |"safe" | tcp_ttl | udp_ttl |

+--------------------+-------+---------+---------+

AIX n 60 30DEC Pathworks V5 n 30 30

FreeBSD 2.1R y 64 64

HP/UX 9.0x n 30 30

HP/UX 10.01 y 64 64

Irix 5.3 y 60 60

Irix 6.x y 60 60

Linux y 64 64 MacOS/MacTCP 2.0.x y 60 60

OS/2 TCP/IP 3.0 y 64 64

OSF/1 V3.2A n 60 30

Solaris 2.x y 255 255

SunOS 4.1.3/4.1.4 y 60 60

Ultrix V4.1/V4.2A n 60 30

VMS/Multinet y 64 64 VMS/TCPware y 60 64

VMS/Wollongong 1.1.1.1 n 128 30

VMS/UCX (latest rel.) y 128 128

MS WfW n 32 32

MS Windows 95 n 32 32

MS Windows NT 3.51 n 32 32

MS Windows NT 4.0 y 128 128

Assigned Numbers (RFC1700, J. Reynolds, J.Postel, October 1994):

IP TIME TO LIVEPARAMETER

The currentrecommended defaulttime to live (TTL)for the InternetProtocol (IP) is 64.

Safe: TCP and UDPinitial TTL valuesshould be set to a"safe" value of at

least 60 today.

IP’s TTL Time To Live field




• The idea behind the TTL field is that IP packets can not travel around the

Internet forever, from router to router.

• Eventually, the packet’s TTL which reach 0 and be dropped by the router, even

if there is a routing loop somewhere in the network.

Decrement by 1, if 0 drop the

packet.

IP Header

0 15 16 31

4-bitVersion

4-bitHeader

Length

8-bit Type OfService

(TOS)


16-bit Identification

3-bit

Flags 13-bit Fragment Offset

8 bit Time To LiveTTL




Options (if any)

Data


RIP routing process




RIP routing process

• Request for Comments (RFC) 1058

• RIP has evolved over the years from a Classful Routing Protocol, RIPVersion 1 (RIP v1), to a Classless Routing Protocol, RIP Version 2

(RIP v2). RIP v2 enhancements include:

– Ability to carry additional packet routing information.

– Authentication mechanism to secure table updates. – Supports variable length subnet masking (VLSM).

Configuring RIP




Configuring RIP

Configuring RIP




Configuring RIP

RIP and IGRP:

• Classful network statements only• IOS will take subnetted networks but will translate it into

the classful network for the running-config.

Triggered Extensions





• “A router running RIP can be configured to send a triggered updatewhen the network topology changes using the ip rip triggered

command. This command is issued only on serial interfaces at therouter(config-if)# prompt. After updating its routing table due to

a configuration change, the router immediately begins transmittingrouting updates in order to inform other network routers of the change.

These updates, called triggered updates, are sent independently of the

regularly scheduled updates that RIP routers forward.”






Triggered Extensions to RIP

• There were two problems using RIP to connect to a WAN:

– Periodic broadcasting by RIP generally prevented WAN circuits from beingclosed.

– Even on fixed, point-to-point links, the overhead of periodic RIPtransmissions could seriously interrupt normal data transfer because of thequantity of information that hits the line every 30 seconds.

• To overcome these limitations, triggered extensions to RIP cause RIP to sendinformation on the WAN only when there has been an update to the routingdatabase.

• Periodic update packets are suppressed over the interface on which thisfeature is enabled.

interface serial 0

ip rip triggered



The RIPv1 Protocol




RIP MessageData Link Frame

• MAC Source Address

• MAC Destination Address = Broadcast

IP Packet• IP Source Address

• IP Destination Address = Broadcast: 255.255.255.255

• Protocol field = 17 for UDP

UDP Segment• Source Port number field = 520 for RIP Message

RIP Message (Data portion of IP Packet):

• Routes: Network IP Address

• Hops (metric)

Data Link

Frame

Header

IP Packet

Header

UDP

Segment

Header

RIP

Message

The RIPv1 Protocol

Data Link

Frame

IP Packet

Header

UDP

Segment

RIP

Message




• Command: 1 signifying a Request or 2 signifying a Reply

• Version: 1 for RIP v 1 or 2 for RIP v 2

• Address Family Identifier : 2 signifying IP (only exception is for a Request for the Router’s full routing

table, later Semester in RIP v 2)• IP Address: The address of the destination route, which may be a network address, a subnet address

of a host address.

• Metric: Hop count between 1 and 16. Note: With RIP the sending router increases the metric before

sending out the RIP message.

• Note: The routing table knows the next-hop-ip-address (via) from the source IP address of the packet.

Frame

Header

Header Segment

Header

Message

0 7 8 15 16 23 24 31

Command = 1 or 2 Version = 1 Must be zero

Address family identifier (2 = IP) Must be zero

IP Address (Network Address)

Must be zero

Must be zero

Metric (Hops)

Multiple Routes, up to a maximum of 25

Address family identifier (2 = IP) Must be zero

IP Address (Network Address)

Must be zero

Must be zero

Metric (Hops)

RIP v2 message format





• All the extensions to the original protocol are carried in the unused

fields.

• The Address Family Identifier (AFI) field is set to two for IP. The only

exception is a request for a full routing table of a router or host, in

which case it will be set to zero.






• The Route Tag field provides a way to differentiate between internal andexternal routes.

• External routes are those that have been redistributed into the RIP v2.• The Next Hop field contains the IP address of the next hop listed in the IP

Address field.

• Metric indicates how many internetwork hops, between 1 and 15 for a validroute, or 16 for an unreachable route.

Configuring RIP




Configuring RIP

RIP must be enabled and the networks specified. The remaining tasks areoptional. Among these optional tasks are:

• Applying offsets to routing metrics (Not commonly used)

• Adjusting timers• Specifying a RIP version (RIPv1 or RIPv2)

• Enabling RIP authentication

• Configuring route summarization on an interface

•Verifying IP route summarization

• Disabling automatic route summarization (RIPv2)

• Running IGRP and RIP concurrently (Usually, redistributing, not concurrently.)

• Disabling the validation of source IP addresses

• Enabling or disabling split horizon

• Connecting RIP to a WAN

ip classless command



Department of Computer and systems Engineering

p

• IP classless only affects the operation of the forwarding

processes in IOS. IP classless does not affect the way the

routing table is built.• This command concerns classless and classful routing

behavior, which is not the same as classless and classful

routing protocols (later).

• .

Parent and Child Routes




RouterB#show ip route

172.16.0.0/24 is subnetted, 3 subnets

R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0

C 172.16.2.0 is directly connected, Serial0C 172.16.3.0 is directly connected, FastEthernet0

C 192.168.1.0/24 is directly connected, Serial1

S 172.0.0.0/8 is directly connected, Serial1


S* 0.0.0.0/0 is directly connected, Serial1

Parent Route

• Created automatically whenever there is a route with a mask greaterthan the classful mask.

• For non-VLSM routes, contains the mask of the child routes.

Child Routes

• Routes with masks greater than the default classful mask.

Lookup what?




p



R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0






Routing Table process matches:

• The routing table process compares the left-most bits in the packet’sdestination IP address with the left-most bits in the route in the routing table,looking for a longest-bit-match.

• The subnet mask of the route in the routing table specifies the minimumnumber of left-most bits that must match.

• Before checking child routes, the classful mask of the parent route is used.

• For child routes the parent route’s mask is used.

• For VLSM routes, the mask is contained with the child route.

Classful Routing Behavior




g



R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0






DA = 172.16.4.1

Router(config)# no ip classless

• With classful routing behavior , if the child routes are checked but

there are no matches, the routing lookup process ends and the Packetis dropped. (The packets get in, but they can’t get out!)

• Supernet and default routes are not checked.

• Default with IOS 11.2 and prior

Classless Routing Behavior






R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0






DA = 172.16.4.1

Router(config)# ip classless

• With classless routing behavior , if the child routes are checked but

there are no matches, the routing lookup process continues with otherroutes in the routing table, including supernet and default routes.

• 8 bits of 172.0.0.0/8 do match, so this route is used!

• Default with IOS 11.3 and later

Common RIP Configuration Issues




Split Horizon

• The following command is used to disable split horizon:

GAD(config-if)#no ip split-horizon

• The following command is used to enable (default) split horizon:

GAD(config-if)#ip split-horizon





Holddown Timer

• The ideal setting would be to set the timer just longer that the longest

possible update time for the internetwork.

• To change the holddown timer:Router(config-router)#timers basic update invalidholddown flush [sleeptime]





Update Timer

• The default RIP update interval in IOS is 30 seconds. This can be

configured for longer intervals to conserve bandwidth, or for shorter

intervals to decrease convergence time.• To change the update internal:

GAD(config-router)#update-timer seconds




• By default, the IOS software receives RIP Version 1 and Version 2

packets, but sends only Version 1 packets.

• The network administrator can configure the router to only receive and

send Version 1 packets or the administrator can configure the router to

send only Version 2 packets.

Compatibility with RIP v1




NewYork

interface fastethernet0/0

ip address 192.168.50.129 255.255.255.192

ip rip send version 1

ip rip receive version 1


ip address 172.25.150.193 255.255.255.240

ip rip send version 1 2


ip address 172.25.150.225 225.255.255.240

router rip

version 2network 172.25.0.0

network 192.168.50.0

• Interface FastEthernet0/0 isconfigured to send and receiveRIP v1 updates.

• FastEthernet0/1 is configuredto send both version 1 and 2

updates.• FastEthernet0/2 has no special

configuration and thereforesends and receives version 2by default.

RIPv2

Verifying RIP configuration




Verifying RIP configuration




• Also: show running-config

Troubleshooting RIP update issues




Troubleshooting RIP update issues




Other commands to troubleshoot RIP:

• show ip rip database

• show ip protocols {summary}

• show ip route• debug ip rip {events}

• show ip interface brief

Load balancing with RIP




• RIP is capable of load balancing over as many as six equal-cost paths,

with four paths being default. RIP performs what is referred to as

“round robin” load balancing.

• This means that RIP takes turns forwarding packets over the parallelpaths.

• This is only part of the story…



Fast Switching

– Per Destination Load Balancing




• The default for most interfaces is Fast Switching.

• Load balancing is distributed according to the destination IP address.

• Given two paths to the same network, all packets for one destination IPaddress will travel over the first path, all packets for a second destination willtravel over the second path, all packets for the third destination will again travelover the first path, and so on.

• To enable fast switching:

Router(config-if)# ip route-cache

• To enable distributed or process switching:

Router(config-if)# no ip route-cache

ping 10.0.0.1 ping 10.0.0.2


Fast Switching

– Per Destination Load Balancing




Fast Switching

1. Router switches first packet to a particular destination, a routing table lookupis performed and an exit interface is selected.

2. The necessary data-link information to frame the packet for the selectedinterface is retrieved including any ARP cache information.

3. The route and data-link information is stored in fast switching cache.

4. The router uses the cache to look up subsequent packets.

5. All other packets to the same destination are immediately switched out thesame interface without the router performing another routing table lookup,including any recursive lookups. (Also no ARP cache lookup).

ping 10.0.0.1 ping 10.0.0.2


Process Switching

– Per Packet Load Balancing




ping 10.0.0.1 ping 10.0.0.2

Router(config-if)#no ip route-cache

Process Switching• Given equal cost paths, per packet load sharing means that one packet to a

destination is sent over one link, the next packet to the same destination issent over the next link, and so on.

• If the paths are unequal cost, the load balancing may be one packet over the

higher-cost link for every three packets over the lower-cost link, or similarratio.

• With process switching, for every packet, the router performs a route tablelookup and selects an interface, and looks up the data-link information.

• To enable distributed or process switching:

Router(config-if)# no ip route-cache

Which one?




ping 10.0.0.1 ping 10.0.0.2

Router(config-if)#no ip route-cache

Fast Switching or Process Switching

• Process switching (per packet load balancing) has a price, load

balancing may be distributed more evenly but the lower switching time

and processor utilization of fast switching are lost.

ping 10.0.0.1 ping 10.0.0.2


Fast Switching Process Switching

Using debug ip packet with

Fast Switching and Process Switching




• debug ip packet can be used to observe packets sent

and received and the interfaces that are involved.

• IMPORTANT: The debug ip packet command allows

only process switched packets to be observed. Fast switchpackets are not displayed (except for the first packet in the

flow).

Router# debug ip packet

IP: s=192.168.3.2 (FastEthernet0), d=10.0.0.1 (Serial0/0),

g=192.168.1.2, forward


g=192.168.2.2, forward

IP: s=192.168.3.2 (FastEthernet0), d=10.0.0.1 (Serial0/0),g=192.168.1.2, forward


g=192.168.2.2, forward

Load balancing across multiple paths




• By default, most IP routing protocols install a maximum of four parallel

routes in a routing table.

• Static routes always install six routes.

• The exception is BGP, which by default allows only one path to adestination.

• The range of maximum paths is one to six paths. To change the

maximum number of parallel paths allowed, use the following

command in router configuration mode:

Router(config-router)# maximum-paths [number ]

RIP and Administrative Distance



Redistribute Static




• Redistributes static routes into the dynamic routing domain.

• 172.16.0.0/16 will be seen by other RIP routers as a

dynamic route learned via RIP.• The default metric is 0, so B and D will have a hop count of

1, where C will have a hop count of 2.

RIP

172.16.0.0/16

RouterA ip route 172.16.0.0 255.255.0.0 eth 0

Router rip

redistribute static

network ….

SanJose2

hostname SanJose2

Scenario 1: Running RIPv1 on classful networks




interface ethernet 0ip add 192.168.1.1 255.255.255.0

interface serial 0

ip add 192.168.2.1 255.255.255.0

SanJose1

hostname SanJose1interface ethernet 0

ip add 192.168.3.1 255.255.255.0

interface serial 0

ip add 192.168.2.2 255.255.255.0

interface serial 1

ip add 192.168.4.2 255.255.255.0

Baypointe

hostname Baypointe

interface ethernet 0

ip add 192.168.5.1 255.255.255.0

interface serial 0

ip add 192.168.4.1 255.255.255.0





Objective: Running RIPv1 on classful networks

This scenario is the same one we used in the network discovery lab, with the sameconfigurations and the same outputs. The concepts specific to this scenario will becomemore clear when we view the differences between this scenario and Scenario 2: Running

RIPv1 on subnets and between classful networks.

Step 1 – Configuring RIP

First, lets enable RIP on each router.

From global configuration you will enter the command (the default is RIPv1):

Router(config)#router rip

Once you are in the Router RIP configuration sub-mode, all you need to do is enter theclassful network address for each directly connected network, using the networkcommand.

Router(config-router)#network directly-connected-classful-network-address





Here are the commands for each router:

SanJose2#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

SanJose2(config)#router rip

SanJose2(config-router)#network 192.168.1.0SanJose2(config-router)#network 192.168.2.0

Baypointe#configure terminal


Baypointe(config)#router ripBaypointe(config-router)#network 192.168.4.0

Baypointe(config-router)#network 192.168.5.0




SanJose1(config-router)#network 192.168.2.0





LISTENING FOR RIP MESSAGES





LISTENING FOR RIP MESSAGES

Routers will also listen for RIP messages on each interface belonging to one of thenetwork statements.

For example, SanJose2 to will listen for RIP update messages on Ethernet 0because that interface has an IP address that belong to the network192.168.1.0, and also listen for RIP update messages on Serial 0 because thatinterface has an IP address that belongs to the network 192.168.2.0.

As RIP messages are received router, will add those networks in the messages totheir routing tables:

If the RIP message contains a network not currently in the routing table.

If the RIP message contains a network with a better metric (fewer hops) than anentry currently in the routing table.

SanJose201:10:56: RIP: received v1 update from 192.168.2.2 on Serial0

01:10:56: 192.168.4.0 in 1 hops

01:10:56: 192.168.3.0 in 1 hops





Step 3 – Viewing the debug ip rip output and the routing tables

Remember that SanJose1 will learn routes to networks from SanJose2. It

will then send that information to Baypointe, telling Baypointe that it isthe next hop to get to those networks, and incrementing the metric (hop

count) by one.

After convergence, each router will continue to send its RIP updatemessages out the appropriate interfaces every 30 seconds.

Lets look at the debug messages and the routing table for each router:

SanJose2

01:30:45: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (192.168.1.1)

01:30:45: network 192.168.4.0, metric 2

01:30:45: network 192.168.5.0, metric 3

01:30:45: network 192 168 2 0 metric 1




01:30:45: network 192.168.2.0, metric 101:30:45: network 192.168.3.0, metric 2

01:30:45: RIP: sending v1 update to 255.255.255.255 via Serial0 (192.168.2.1)

01:30:45: network 192.168.1.0, metric 1

SanJose2#

01:30:50: RIP: received v1 update from 192.168.2.2 on Serial0

01:30:50: 192.168.4.0 in 1 hops

01:30:50: 192.168.5.0 in 2 hops

01:30:50: 192.168.3.0 in 1 hops

SanJose2#

SanJose2#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

<omitted>

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

U - per-user static route, o - ODR

Gateway of last resort is not set

R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:10, Serial0

R 192.168.5.0/24 [120/2] via 192.168.2.2, 00:00:10, Serial0C 192.168.1.0/24 is directly connected, Ethernet0


R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:10, Serial0

SanJose2#

SanJose1


01:33:05: 192.168.5.0 in 1 hops

SanJose1#


01:33:07: 192 168 1 0 in 1 hops




01:33:07: 192.168.1.0 in 1 hops01:33:08: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (192.168.3.1)

01:33:08: network 192.168.4.0, metric 1

01:33:08: network 192.168.5.0, metric 2

01:33:08: network 192.168.1.0, metric 2

01:33:08: network 192.168.2.0, metric 1

01:33:08: RIP: sending v1 update to 255.255.255.255 via Serial0 (192.168.2.2)01:33:08: network 192.168.4.0, metric 1

01:33:08: network 192.168.5.0, metric 2

01:33:08: network 192.168.3.0, metric 1


01:33:08: network 192.168.1.0, metric 2

01:33:08: network 192.168.2.0, metric 101:33:08: network 192.168.3.0, metric 1



<omitted>



R 192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:12, Serial1

R 192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:10, Serial0


C 192.168.3.0/24 is directly connected, Ethernet0

Baypointe


01:34:53: network 192.168.4.0, metric 1

01:34:53: network 192.168.1.0, metric 3

01:34:53: network 192.168.2.0, metric 2

01:34:53: network 192.168.3.0, metric 2




01:34:53: network 192.168.3.0, metric 201:34:53: RIP: sending v1 update to 255.255.255.255 via Serial0 (192.168.4.1)

01:34:53: network 192.168.5.0, metric 1

Baypointe#


01:34:56: 192.168.1.0 in 2 hops

01:34:56: 192.168.2.0 in 1 hops01:34:56: 192.168.3.0 in 1 hops

Baypointe#show ip route


D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP






R 192.168.1.0/24 [120/2] via 192.168.4.2, 00:00:23, Serial0

R 192.168.2.0/24 [120/1] via 192.168.4.2, 00:00:23, Serial0

R 192.168.3.0/24 [120/1] via 192.168.4.2, 00:00:23, Serial0

NOTE: At this point all routers should be able to ping all networks We will discuss RIP much





NOTE: At this point all routers should be able to ping all networks. We will discuss RIP muchmore in the chapter on Routing Protocols (RIP).

Step 4 – Turning-off debug

Don’t forget to turn-off debug when you are done collecting the output.

Router# undebug all

or

Baypointe# undebug ip rip

Step 5 – Reflections

• For each router compare the RIP received messages with its routing table. Now you seehow the information is entered into the routing table.

• Cisco IOS uses split horizon with poison reverse, however this information is notdisplayed with debug ip rip command.

• You will notice that the routers send RIP messages out their stub Ethernet interfaces,even though there are no routers out there to receive those messages. This does take upunnecessary bandwidth on the link; so later we will see how to keep those RIP messagesfrom going out those interfaces.

SanJose2

hostname SanJose2


Scenario 2: Running RIPv1 on subnets and between

classful networks




ip add 172.30.1.1 255.255.255.0

interface serial 0

ip add 172.30.2.1 255.255.255.0

SanJose1

hostname SanJose1interface ethernet 0

ip add 172.30.3.1 255.255.255.0

interface serial 0

ip add 172.30.2.2 255.255.255.0

interface serial 1

ip add 192.168.4.9 255.255.255.252

Baypointe

hostname Baypointe


ip add 192.168.5.1 255.255.255.0

interface serial 0

ip add 192.168.4.10 255.255.255.252

Objective: Running RIPv1 on subnets and between classful networks


classful networks




Objective: Running RIPv1 on subnets and between classful networks

In this scenario we will see how subnetted routes are distributed with the same classful

network. We will also see how RIPv1 automatically summarizes between classful

network boundaries. You will notice that SanJose1 and SanJose2 have subnets

belonging to the 172.30.0.0 network, but Baypointe does not.

Making changes between Scenario 1 and Scenario 2

Be sure to change the IP addressing as displayed in the diagram and Basic Configuration

section for Scenario 2. Sometimes when changing the IP address on a serial

interface, you may need to reset that interface by doing a shutdown, wait for the

LINK-5-CHANGED message, then follow it with a no shutdown command.

If you have just completed Scenario 1, lets remove RIP by issuing the following command

on each router :

Router(config)# no router rip

Step 1 Configuring RIP


classful networks




Step 1 – Configuring RIP

Once again, lets enable RIP on each router.

Once you are in the Router RIP configuration sub-mode, all you need to do

is enter the classful network address for each directly connected

network, using the network command. If a router has multiple

interfaces on the same classful network, you will only need to enter a

single command enabling RIP on all interfaces for that network.

Router(config-router)#network directly-connected-classful-network-address








p


Notice we only used a single network statement for SanJose2, which includes both interfaces, on different

subnets, of the 172.30.0.0 major network.






Again, notice that we only used a single network statement for SanJose1, which includes both interfaces, on

different subnets, of the 172.30.0.0 major network.

Baypointe#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.Baypointe(config)#router rip



Question: What would happen if you entered a network statement that


classful networks




Question: What would happen if you entered a network statement that

was a subnet? For example:



Answer: The IOS would automatically convert it to a classful network

statement:

SanJose2#show running-config

router rip

network 172.30.0.0

Step 2 – Viewing the debug ip rip output and the routing tables

SanJose2

SanJose2# debug ip rip


00:14:10: 172.30.3.0 in 1 hops

00:14:10: 192.168.4.0 in 1 hops

00 14 10 192 168 5 0 i 2 h




00:14:10: 192.168.5.0 in 2 hops

SanJose2#


00:14:29: subnet 172.30.2.0, metric 1

00:14:29: subnet 172.30.3.0, metric 2

00:14:29: network 192.168.4.0, metric 2

00:14:29: network 192.168.5.0, metric 3


00:14:29: subnet 172.30.1.0, metric 1

SanJose2#


00:14:39: 172.30.3.0 in 1 hops

00:14:39: 192.168.4.0 in 1 hops00:14:39: 192.168.5.0 in 2 hops

SanJose2# undebug all



<omitted>

Gateway of last resort is not set172.30.0.0/24 is subnetted, 3 subnets

C 172.30.2.0 is directly connected, Serial0

R 172.30.3.0 [120/1] via 172.30.2.2, 00:00:08, Serial0

C 172.30.1.0 is directly connected, Ethernet0

R 192.168.4.0/24 [120/1] via 172.30.2.2, 00:00:08, Serial0

R 192.168.5.0/24 [120/2] via 172.30.2.2, 00:00:08, Serial0

Reflections


classful networks




• IMPORTANT INFORMATION: RIPv1 is a classful routing protocol. Classful routing

protocols do not send the subnet mask with network in routing updates, ie. 172.30.1.0 is

sent by SanJose1 to SanJose2 without any subnet mask information.

• QUESTION: Notice that SanJose2 is receiving the subnet 172.30.3.0 from SanJose1,

which is put in the routing table under the parent network (classful network) of 172.30.0.0with the /24 subnet mask (172.30.0.0/24 is subnetted, 3 subnets). Also notice that the

RIP message received from SanJose1 was “172.30.3.0 in 1 hops” but did not include a

subnet mask for the subnet. How does SanJose2 know that this subnet has a /24

(255.255.255.0) subnet mask?

• ANSWER: SanJose2 received this information on an interface belonging to the sameclassful network as the incoming 172.30.3.0 update. The IP address that SanJose1

received the “172.30.3.0 in 1 hops” message was on (Serial 0) with an IP address of

172.30.2.1 and a subnet mask of 255.255.255.0. SanJose2 uses its own subnet mask

and applies it to this and all other 172.30.0.0 subnets it receives on this interface. The

172.30.3.0 network is placed with the other 172.30.0.0 /24 subnets in the routing table.

• Routers running RIPv1 are limited to using the same subnet mask for all subnets with the

same classful network. Classless routing protocols like RIPv2 allow the same major

(classful) network to use different subnet masks on different subnets. This is known as

VLSM (Variable Length Subnet Masks) and is discussed later.

SanJose1

SanJose1#debug ip rip

RIP protocol debugging is on

SanJose1#


00:17:52: subnet 172.30.2.0, metric 1

00:17:52: subnet 172.30.1.0, metric 2

00 17 52 t k 192 168 4 0 t i 1




00:17:52: network 192.168.4.0, metric 1

00:17:52: network 192.168.5.0, metric 2


00:17:52: subnet 172.30.3.0, metric 1

00:17:52: network 192.168.4.0, metric 1


00:17:52: network 172.30.0.0, metric 1

SanJose1#


00:18:10: 172.30.1.0 in 1 hops

SanJose1#

00:18:12: RIP: received v1 update from 192.168.4.10 on Serial100:18:12: 192.168.5.0 in 1 hops




<omitted>


172.30.0.0/24 is subnetted, 3 subnetsC 172.30.2.0 is directly connected, Serial0


R 172.30.1.0 [120/1] via 172.30.2.1, 00:00:14, Serial0



R 192.168.5.0/24 [120/1] via 192.168.4.10, 00:00:10, Serial1

Reflections


classful networks




• The same subnet route information applies with routes sent from

SanJose2 to SanJose1 (see Reflections for SanJose2).

• SanJose1 knows that the 172.30.1.0 update has a subnet mask of /24

because it received it on an interface with a /24 subnet mask (Serial 0,172.30.3.2 255.255.255.0).



SanJose1#


00:17:52: subnet 172.30.2.0, metric 1

00:17:52: subnet 172.30.1.0, metric 200:17:52: network 192.168.4.0, metric 1




00: :5 : et o 9 . 68. .0, et c

00:17:52: network 192.168.5.0, metric 2


00:17:52: subnet 172.30.3.0, metric 1

00:17:52: network 192.168.4.0, metric 1

00:17:52: network 192.168.5.0, metric 2


00:17:52: network 172.30.0.0, metric 1

SanJose1#


00:18:10: 172.30.1.0 in 1 hops

SanJose1#


00:18:12: 192.168.5.0 in 1 hops



Codes: <omitted>




R 172.30.1.0 [120/1] via 172.30.2.1, 00:00:14, Serial0



R 192.168.5.0/24 [120/1] via 192.168.4.10, 00:00:10, Serial1

More ReflectionsIMPORTANT INFORMATION N ti th RIP d t b i t t S i l 1


classful networks




• IMPORTANT INFORMATION: Notice the RIP update being sent out Serial 1:RIP: sending v1 update to 255.255.255.255 via Serial1 (192.168.4.9)

network 172.30.0.0, metric 1

• Compare that to the same information for the 172.30.0.0 network being sent outSerial 0 & Ethernet 0:

RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.2)

subnet 172.30.3.0, metric 1

• Notice that the 172.30.0.0 subnets are being summarized to their classfulnetwork address of 172.30.0.0 when sent out Serial 1 to Baypointe.

• RIP automatically summarizes RIP updates between classful networks.Because the 172.30.0.0 update is being sent out an interface (Serial 1) on adifferent classful network (192.168.4.0), RIP sends out only a single update for

the entire classful network instead of all of the different subnets. This is similarto what we did with summarizing several static routes into a single static route.

• A router like SanJose1, which has an interface in more than one classfulnetwork is sometimes called a “boundary router” in RIP. Boundary routersautomatically summarize RIP subnets from one classful network to the other.

More Reflections (continued)


classful networks




• How is this an advantage? Fewer updates sent and received, resulting in less

bandwidth used for routing updates between SanJose1 and Baypointe. Just as

importantly, Baypointe will now only have a single route for the 172.30.0.0/16

network, no matter how many subnets there are or how it is subnetted. This willresult in faster lookup process in the routing table for Baypointe.

• What do you expect to see in Baypointe’s received RIP messages and its

routing table? That’s right, only a single 172.30.0.0 network via SanJose1.

• Are there any disadvantages? Yes, discontinguous networks. We will see

this later, but the idea here is what if Baypointe had another connection viaSerial 1 to another router, SantaCruz1 on 192.168.4.12/30 subnet, which also

has other 172.30.0.0/24 subnets (172.30.4.0/24, 172.30.5.0/24, etc.).

Baypointe would also receive the same 172.30.0.0 network from SantaCruz1 as

well. Baypointe would not know how to reach the specific subnet, and

mistakenly load-balance the packets between the two routers. We will see anexample of this later this semester.

Baypointe

Baypointe#debug ip rip


Baypointe#


00:20:09: 172 30 0 0 in 1 hops




00:20:09: 172.30.0.0 in 1 hops

Baypointe#


00:20:24: network 172.30.0.0, metric 2


00:20:24: network 192.168.5.0, metric 1

Baypointe#

Baypointe#undebug all

Baypointe#show ip route


<omitted>


R 172.30.0.0/16 [120/1] via 192.168.4.9, 00:00:11, Serial0




Reflections

• N ti th t B i t i l i i th l f l f th 172 30 0 0


classful networks




• Notice that Baypointe is only receiving the classful summary of the 172.30.0.0

subnets:

RIP: received v1 update from 192.168.4.9 on Serial0

172.30.0.0 in 1 hops• SanJose1 automatically summarized the subnets into a single classful update.

• This keeps Baypointe’s routing table smaller, resulting in faster routing table

lookups.

• This also isolates any changes in the 172.30.0.0 network on SanJose1 and

SanJose2 from affecting Baypointe. In other words, SanJose1 and SanJose2can add and delete 172.30.0.0/24 subnets without affecting Baypointe’s routing

table, as Baypointe doesn’t care. Baypointe will send all packets destined for

the 172.30.0.0/16 network to SanJose1. Baypointe’s routing table:

R 172.30.0.0/16 [120/1] via 192.168.4.9, 00:00:11, Serial0

Also, the subnet mask scheme could be changed (i.e. to /27) on the 172.30.0.0

network without affecting Baypointe’s routing table or the RIP update sent to

Baypointe by SanJose1.

Scenario 3: Running RIPv1 on a stub network

SanJose2

hostname SanJose2


ip add 172.30.1.1 255.255.255.0

interface serial 0




interface serial 0

ip add 172.30.2.1 255.255.255.0

SanJose1

hostname SanJose1


ip add 172.30.3.1 255.255.255.0

interface serial 0

ip add 172.30.2.2 255.255.255.0

interface serial 1

ip add 192.168.4.9 255.255.255.252

Baypointe

hostname Baypointe


ip add 192.168.5.1 255.255.255.0

interface serial 0

ip add 192.168.4.10 255.255.255.252

Objective: Running RIPv1 on a stub network

In this scenario we will modify Scenario 2 to only run RIP between SanJose1 and SanJose2. Scenario 3 is a

very common situation for many companies. It is very common that a company will want to run a

dynamic routing protocol (RIPv1 in our case) within their own network, but find in unnecessary to run a

dynamic routing protocol between their company and their ISP.




For Scenario 3 let us assume that Baypointe is the ISP for our Company XYZ, which consists of the

SanJose1 and SanJose2 routers using the 172.30.0.0/16 major network, subnetted with a /24 mask.

Company XYZ is a stub network, meaning there is only one way in and out of the 172.30.0.0/16 network, in

via SanJose1 (a.k.a. the entrance router) and out via Baypointe (the ISP). It is doesn’t make sense for

SanJose1 to send Baypointe the RIP update of 172.30.0.0 every 30 seconds, because Baypointe has no

other way to get there. RIP update message from SanJose1 to Baypointe, if RIP were configured:

RIP: received v1 update from 192.168.4.9 on Serial0

172.30.0.0 in 1 hops

Instead, it makes more sense for Baypointe to have a static route configured for the 172.30.0.0/16 network viaSanJose1.

Well, how about traffic from Company XYZ towards the Internet? It makes no sense for Baypointe to send

more than the 120,000 summarized Internet routes to SanJose1. All SanJose1 needs to know is that if it

is not in the 172.30.0.0 network then send it to the ISP, Baypointe. This is the same for all other

Company XYZ routers (only SanJose2 in our case), that they would send all traffic with destination IPaddresses other than 172.30.0.0 to SanJose1 who would forward them on to Baypointe. Let’s see how

to configure this.

Making changes between Scenario 2 and Scenario 3

Be sure to change the IP addressing as displayed in the diagram and Basic

Configuration section for Scenario 3. Sometimes when changing the IP addresson a serial interface, you may need to reset that interface by doing a shutdown,




y y y g

wait for the LINK-5-CHANGED message, then follow it with a no shutdown

command.

If you have just completed Scenario 2, lets remove RIP by issuing the following

command on each router :

Router(config)# no router rip

Step 1 – Configuring RIP on SanJose1 and SanJose2












SanJose1(config)#router ripSanJose1(config-router)#network 172.30.0.0

Notice that we are only including the 172.30.0.0 interfaces, networks, for SanJose1.

We will not be exchanging RIP updates with Baypointe via the 192.168.4.0/30

network.

Step 2 - Configuring the default static route on SanJose1

On SanJose1, let’s configure a static default route, sending all default traffic, packets with

destination IP addresses which do not match a specific route in the routing table, to

Baypointe.

SanJose1(config)# ip route 0 0 0 0 0 0 0 0 serial 1




SanJose1(config)# ip route 0.0.0.0 0.0.0.0 serial 1

Notice, since the exit interface is a point-to-point serial interface we chose to use the exit-

interface instead of a intermediate-address (next-hop-ip address), saving the router from

having to do a recursive lookup. However, using an intermediate-address (next-hop-ip-

address) would have worked also.

Previous to IOS version 12.1, SanJose1 would propagate, send, this default route

automatically via RIP with its RIP updates to all other routers (in this case SanJose2).

SanJose2 and all other routers will receive this default route via RIP and forward to all

other routers in the RIP routing domain.

However, with IOS 12.1 and later, we need to enter the default-information originate

command on Baypointe, the router with the static default route. This will tell SanJose1 to

include the static default route with its RIP updates to SanJose2.SanJose1(config)#router rip

SanJose1(config-router)#default-information originate

Step 3 - Configuring the static route on Baypointe for the 172.30.0.0/16 network

Since Baypointe and SanJose1 are not exchanging RIP updates, we need to configure a static

route on Baypointe for the 172.30.0.0/16 network. This will send all 172.30.0.0/16 traffic,

packets with destination IP addresses of 172.30.x.x, to SanJose1.




Baypointe(config)# ip route 172.30.0.0 255.255.0.0 serial 0

Once again, notice, since the exit interface is a point-to-point serial interface we chose to usethe exit-interface instead of a intermediate-address (next-hop-ip address), saving the

router from having to do a recursive lookup. However, using an intermediate-address

(next-hop-ip-address) would have worked also.

SanJose1



SanJose1#


02:09:10: 172.30.1.0 in 1 hops

SanJose1#





02:09:29: subnet 172.30.2.0, metric 1

02:09:29: subnet 172.30.1.0, metric 2

02:09:29: default, metric 1


02:09:29: subnet 172.30.3.0, metric 1

02:09:29: default, metric 1

SanJose1#

SanJose1#undebug all

SanJose1#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

<omitted>

Gateway of last resort is 0.0.0.0 to network 0.0.0.0


C 172.30.2.0 is directly connected, Serial0C 172.30.3.0 is directly connected, Ethernet0

R 172.30.1.0 [120/1] via 172.30.2.1, 00:00:13, Serial0




Reflections

• Notice that the static default route is being propagated by SanJose1 to





• Notice that the static default route is being propagated by SanJose1 to

other routers (SanJose2) via RIP.

• Notice the static route in the routing table and the “Gateway of last

resort.”

SanJose2



SanJose2#


02:07:06: 172.30.3.0 in 1 hops

02:07:07: 0.0.0.0 in 1 hops

S J 2#




SanJose2#


02:07:23: subnet 172.30.2.0, metric 1

02:07:23: subnet 172.30.3.0, metric 2

02:07:23: default, metric 202:07:23: RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.1)

02:07:23: subnet 172.30.1.0, metric 1

SanJose2#

SanJose2#undebug all

SanJose2#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

<omitted>



Gateway of last resort is 172.30.2.2 to network 0.0.0.0


R 172.30.3.0 [120/1] via 172.30.2.2, 00:00:22, Serial0


R* 0.0.0.0/0 [120/1] via 172.30.2.2, 00:00:22, Serial0

Reflections

• Notice that SanJose2 is receiving the default route from SanJose1





Notice that SanJose2 is receiving the default route from SanJose1.

• SanJose2 forwards that default route out Ethernet 0, a RIP enabled

interface, although there are no other routers on that segment.

• Notice the default route in the routing table and that it was learned via

RIP.

• Notice the “Gateway of last resort”

Baypointe

No RIP messages, as we are not running RIP.

Baypointe#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D EIGRP EX EIGRP external O OSPF IA OSPF inter area




D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate defaultU - per-user static route, o - ODR






Reflections

• Notice that RIP is not being used on Baypointe. The only routes that arenot directly-connected is the static route.

show ip protocols command

SanJose2 router from Scenario 3.





SanJose2#show ip protocols

Routing Protocol is "rip"

Sending updates every 30 seconds,next due in 11 secondsInvalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is

Incoming update filter list for all interfaces is

Redistributing: rip

Default version control: send version 1, receive any version

Interface Send Recv Key-chain

Ethernet0 1 1 2Serial0 1 1 2

Routing for Networks:

172.30.0.0

Routing Information Sources:

Gateway Distance Last Update

172.30.2.2 120 00:00:04

Distance: (default is 120)SanJose2#

Be sure to understand this command. We will examine it again when we take a closer look at RIPv1,RIPv2 and IGRP. Take a look at the items in bold and make sure you understand them.

A Few Final Notes

RIP uses broadcasts

• Notice that RIPv1 sends out its RIP updates via an IP broadcast.02:07:23: RIP: sending v1 update to 255.255.255.255 via Ethernet0

(172.30.1.1)




All devices on the segment will see these RIP updates.

The passive-interface command• How can you keep a RIP update from being sent out an interface which does not have any

other routers? (i.e The Ethernet interfaces in our network.)

• Because the network statement includes all interfaces which have an IP address on that

classful network, by default RIP will send out updates out each one of those interfaces.

• Do keep RIP from sending updates out an interface which does not have any other routers,you can use the passive-interface command.

• The passive-interface command allows the interface to receive RIP updates on the

interface, but does not send RIP updates out that interface.

• For example, to keep SanJose2 from sending out RIP updates out Ethernet 0, you can do

the following:SanJose2(config)#router rip


SanJose2(config-router)# passive-interface Ethernet 0



How can I remove a single network from RIP?

Instead of using the following command to remove all networks from RIP:

Router(config)#no router rip

You can specify just the network you wish to remove by using the no network command, for




p y j y y g

example:

Router(config)#router rip

Router(config-router)#no network 172.30.0.0

Debug ip routing - FYI

• If you wish to see what is happening in the router’s routing table process, you can use

the debug ip routing command:

SanJose2#debug ip routing

IP routing debugging is on

SanJose2#conf t


SanJose2(config)#router ripSanJose2(config-router)#network 172.30.0.0

SanJose2(config-router)#

00:15:03: RT: add 172.30.3.0/24 via 172.30.2.2, rip metric [120/1]

00:15:03: RT: add 0.0.0.0/0 via 172.30.2.2, rip metric [120/1]

00:15:03: RT: default path is now 0.0.0.0 via 172.30.2.2

00:15:03: RT: new default network 0.0.0.0

DistanceVectorRouting_5

Documents

Transcript of DistanceVectorRouting_5