1 CCNA 3 v3.1 Module 6. 2 CCNA 3 Module 6 Switch Configuration.
1 CCNA 2 v3.1 Module 7. 2 CCNA 2 Module 7 Distance Vector Routing Protocols RIP and IGRP.
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Transcript of 1 CCNA 2 v3.1 Module 7. 2 CCNA 2 Module 7 Distance Vector Routing Protocols RIP and IGRP.
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Distance vector routing updates
• Routing table updates occur periodically or when topology change
• Routers send their entire routing table to all adjacent neighbors
• Routing table contains info about routing metrics
Hop count
Bandwidth
Cost
Load
Reliability
Delay
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Distance vector routing loop
• Loops occur when inconsistent routing tables are not updated due to slow convergence in a changing network
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• Count to infinity
Packets loop continuously around the network
Packet never reach their destination
May occur because
Network is down
Interface not configured correctly
• To overcome count to infinity
Specify a maximum hop count
Split Horizon
Route Poisoning
Hold Down timers
Count To Infinity
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• Used to overcome count to infinity
• Hop count
The number of routers a packet goes through
It increases each time a packet goes through a router
• RIP maximum hop count is 15
Packet is discarded if the hop count of 16 is reached
This is because the network 1 is unreachable
Maximum hop count
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• Solution to count to infinity
• Do not send routing updates to the router you learned the information about
Reduces incorrect routing information
Reduces routing overhead
Split Horizon
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• Poison reverse
• 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.
• Failed routes are advertised with infinite metrics.
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Avoiding routing loops with triggered updates
• Triggered updates
Sent immediately when topology changes
Used in conjunction with route poisoning
Ensure all routers know of failed routes before any holddown timers can expire
i.e., Don’t wait for the time limit to expire
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Preventing routing loops with holddown timers
• When a router receives an update saying a route is down, it marks the route as inaccessible and starts a hold-time timer
• If at any time before the holddown timer expires an update is received from:the same neighbour indicating the network is accessible again
Router marks network as accessible + removes holddown timer
A different neighbour with a better metric
Router marks network as accessible + removes holddown timer
A different neighbour with a poorer metric
The update is ignored
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RIP Routing Protocol
• Open standard RFC 1058 and Internet Standard (STD) 56
• Documented by:
Request for Comments (RFC) 1058
Internet Standard (STD) 56.
• RIP Version 1 (RIP v1)
Classful Routing Protocol
• RIP Version 2 (RIP v2)
Classless Routing Protocol
Ability to carry additional packet routing information.
Authentication mechanism to secure table updates.
Supports variable length subnet masking (VLSM)
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• RIP has slow convergence which results in
Routing loops
Count to infinity
• To reduce these RIP uses
Count-to-infinity
Split horizon
Poison reverse
Holddown counters
Triggered updates
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Configuring RIP
• Enter global configuration mode
Router#config t
• Specify RIP as the routing protocol
Router(config)#router rip
• Specify directly connected networks
Router(config-router)#network 10.10.10.0
• A router running RIP can be configured to
Send triggered updates independently the scheduled updates (every 30 seconds)
Router(config-if)#ip rip triggered
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RIP Configuration Issues• Count-to-infinity
Does not have to be configured with RIP
• To disable Split Horizon
Router(config-if)#no ip split-horizon
• Default holddown for RIP is 180 seconds
• The default RIP update 30 seconds
Can be configured for longer intervals
To conserve bandwidth
Can be configured for for shorter intervals
To decrease convergence time
• Command
Router(config)#router rip
Router(config-router)#update-timer seconds
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• To prevent RIP updates leaving an interface
Router(config-router)#passive-interface Fa0/0
• To configure RIP to exchange routing information in a non-broadcast network (Frame Relay)
RIP needs to be told of other neighboring RIP routers
Router(config-router)#neighbor <ip address>
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•To configure software to send and receive Rip V1 and V2 packets•Router(config-router)#version{1|2}
•To configure an interface to send RIP V1 packets•Router(config-if)#ip rip send version 1
•To configure an interface to send RIP V2 packets•Router(config-if)#ip rip send version 2
•To configure an interface to send RIP V1 and V2 packets•Router(config-if)#ip rip send version 1 2
•To configure and interface to receive RIP V1 packets•Router(config-if)#ip rip receive version 1
•To configure and interface to receive RIP V2 packets
•Router(config-if)#ip rip receive version 2
•To configure and interface to receive RIP V1 and V2 packets
•Router(config-if)#ip rip receive version 1 2
Configure Router to Send/Receive Packets
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Verifying RIP configuration • show ip protocols
Shows which routing protocols are carrying IP traffic on the router
Confirms
RIP routing is configured
The correct interfaces are sending and receiving RIP updates
The router is advertising the correct networks
• show ip route
Displays the routing table
Verify that routes received by RIP neighbours are installed in the routing table
• Additional commands to check RIP configuration
show interface interface
show ip interface interface
show running-config
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Troubleshooting RIP update issues
• debug ip rip
Displays RIP routing updates as they are sent and received
Problems such as discontiguous subnets or duplicate networks can be diagnosed with this command
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Other commands to troubleshoot RIP:
• show ip rip database
• show ip protocols {summary}
• show ip route
• debug ip rip {events}
• show ip interface brief
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Preventing routing updates through an interface
• network command
If it is issued for a given network using RIP or IGRP
router immediately begin sending advertisements out all interfaces within the specified network address range
Router(config)#router RIP
Router(config-router)#network 192.101.21.0
• passive interface command
Stops the router from sending updates through a particular interface to a neighbor
Router continues to listen and use routing updates from that neighbor
Prevent other systems from learning about routes dynamically
Router(config)#router RIP
Router(config-router)#passive-interface fa0/0
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Load balancing with RIP • Allows a router to take advantage of
multiple best paths to a given destination
Calculated by dynamic routing protocol
Entered statically by administrator
• RIP performs “round robin” load balancing
over 6 equal-cost paths, with 4 paths being default
takes turns forwarding packets over the parallel paths
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• Interface pointer cycles through the interfaces and routes in a deterministic fashion such as 1-2-3-4-1-2-3-4-1
• RIP metric is hop count
no regard is given to the speed of the links
56 Kbps path will be given the same preference as the 155 Mbps path
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• When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table
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• When a router has multiple routes to a network
route with lowest admin distance is installed in routing table
• If the router has many routes, learned via the same routing process with the same admin distance
router chooses the path with the lowest cost or metric
• If the router has multiple paths to a destination with the same admin distance and cost
load-balancing can occur
• Cisco IOS allows up to 6 equal cost routes in a routing table
EIGRP allows up to 4 equal cost routes
Static routes always install 6 routes
BGP by default allows only 1 route
IGRP can load balance up to 6 unequal links
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• Router(config-router)#maximum-paths [no.]
• Cisco IOS offers two methods of load balancing:
per-packet:If process switching is enabled,
Router will alternate paths on a per-packet basis
per-destination: If fast switching is enabled
Only one route will be used for that destination address
All packets bound for a specific host take the same path
Packets bound for a different host on the same network may use an alternate route
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Integrating static routes with RIP • Static routes
User-defined routes
Force packets to take a specific path
Useful for specifying a “gateway of last resort”; a default route
• Administrative Distance (AD)
Each dynamic routing protocol has a default AD
A static route can be defined as less desirable than a dynamically learned route, as long as the AD of the static route is higher than that of the dynamic route
• floating static route
A route configured on a router to take the place of the RIP route in the event that the RIP routing process fails
• RIP can advertise static routes using the redistribute static command
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IGRP features • Distance vector routing protocol
• Developed by Cisco
• Sends routing updates every 90 second
Sends entire table
• Advertising networks for an autonomous system
• Metrics used are normally bandwidth and delay
Bandwidth
Delay – amount of time to get to the destination
Reliability – reliability of link determined by exchange of keepalives
Load
MTU – maximum transmission unit of the path
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• IGRP converges faster than RIP
thereby avoiding the routing loops
• IGRP does not share the hop count limitation of RIP.
• Useful Commandsshow interface interface
show running-config
show running-config interface interface
show running-config | begin interface interface
show running-config | begin igrp
show ip protocols
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Metric K1 represents bandwidth by default is set to 1Metric K3 represents delay and by default is set to 1Metric K2, K4 and K5 are set to 0.
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show ip route shows the IGRP metric values in brackets.A link with a higher bandwidth will have a lower metricA route with a lower cumulative delay will have a lower metric
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IGRP Routes• GRP advertises three types of routes:
Interior
Routes between subnets of a network attached to a router interface
If the network attached to a router is not subnetted, IGRP does not advertise interior routes
System
Routes to networks within an autonomous system number
The IOS derives system routes from directly connected network interfaces
System routes information provided by other IGRP routers.
System routes do not include subnet information
Exterior
Routes to networks outside the autonomous system that are considered when identifying a gateway of last resort.
Gateway of last resort is used if
a better route is not found
The destination is not a connected network
Exterior routes are also referred to as "candidate default"
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IGRP stability features
Holddowns
Prevent regular update messages from inappropriately reinstating a route that may not be up
Split horizons
• It is usually not useful to send information about a route back in the direction from which it came
• Prevent routing loops
Poison reverse
• Poison reverse updates then are sent to remove the route and place it in holddown.
• With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.
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IGRP Timers• update timer specifies
how frequently routing update messages should be sent
Default is 90 sec
• invalid timer specifies
how long a router should wait in the absence of routing-update messages about a route before declaring that route invalid
Default is 3 times the update period (270 sec)
• holddown timer specifies
the amount of time for which information about poorer routes is ignored
Default is 3 times the update timer period + 10 sec (280 sec)
• flush timer indicates
how much time should pass before a route is flushed from the routing table
Default is 7 times the routing update timer (after 630 sec)
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Configuring IGRP
• To set IGRP routing on
Router# config t
Router(config)# router igrp 101
Router(config-router)#network 192.101.2.0
• To turn IGRP routing off
Router(config)# no router igrp 101