CCNP RoutingSemester 5

Chapter 3Routing Overview

Protocols and MetricsProtocol

RIPv1

IGRP

EIGRP

OSPF

IS-IS

Metric

Hop count

Bandwidth, delay, load, reliability, MTU

Bandwidth, delay, load, reliability, MTU

Cost (the Cisco default states the cost of an interface is inversely proportional to the bandwidth of the interface. A higher bandwidth indicates a lower cost)

Cost

Protocols and CodesCodeIDEXRCSEBL ???

ProtocolIGRPEIGRPExternal EIGRPRIPConnectedStaticEGPBGPIS-IS

Protocols and CodesL1

L2

M

U

O

T

O

IS-IS Level 1

IS-IS Level 2

Mobile

Per user static route

ODR

Traffic-engineered route

OSPF Networks from within the same area as the router. These are networks learned from router and network LSAs

OSPF Codes

IA ??

N1

N2

OSPF interarea. This is sent out by the ABRs and is created from the summary link (LSA type 3 and 4). These routes will not bee seen on a router within a totally stubby area because it will not receive LSAs external to the area.

OSPF NSSA external type 1

OSPF NSSA external type 2

OSPF Codes

E1

E2

OSPF external type 1 – generated by ASBR and show routes that are external to AS. Cost is summarization of external cost plus the cost of the path to the ASBR. Will not be seen in a stub or totally stubby area.

OSPF external type 2 – these routes do not take into account the cost of the path to the ASBR – consider only external cost

Distance Vector Characteristics• Sends entire routing table at periodic intervals out all

interfaces – will also send triggered updates to reflect changes in network

• Involves updates sent using a broadcast address to everyone on the link

• Uses metric based on how distant the remote network is to the router – IGRP does not conform

• Has knowledge of the network based on information learned from neighbors

• Includes a routing table that is database viewed from perspective of each router

Distance Vector Characteristics

• Uses Bellman Ford algorithm for calculating best path

• Does not consume many router resources – but is heavy in use of network resources

• Maintains one domain in which all routes are known

• Is not restricted by addressing scheme• Involves slower convergence – each routing table

on every intervening router must be updated before changes reach remote end of network

Link State Characteristics

• Sends incremental updates when a change is detected – OSPF will send summary info every 30 minutes, regardless of incremental updates

• Typically involves updates sent to those routers participating in the routing protocol domain, via a multicast address

• Is capable of using a complex metric, referred to as cost

• Has knowledge of network based on info learned from every router in the area

Link State Characteristics• Has topological database that is the same for every

router in the area – table unique to each router• Uses Dijkstra algorithm• Uses many router resources but is relatively low in its

demand for network resources• Has hierarchical design of areas that allow for

summarization and growth• For effective use, the addressing scheme should reflect

hierarchical design of network• Quicker convergence because update is flooded

immediately throughout the network

Differences BetweenRIPv1 and OSPF

RIPv1

• Simple protocol to design, configure and maintain

• Does not require a hierarchical address

• Does not pass the subnet mask in the routing update; can’t handle classless routing or VLSM

OSPF

• Complex to design, configure and maintain

• Requires hierarchical IP addressing scheme if full benefits are to be harnessed

• Carries the mask in the update; can implement VLSM, summarization and classless routing

Differences BetweenRIPv1 and OSPF

RIPv1• Is limited to 15 hops

• Does not acknowledge updates; repeats them periodically every 30 sec.

• Has a routing table that is sent out of every interface every 30 seconds

OSPF• Unlimited in diameter of

network; it is suggested area not exceed 50 networks

• Acknowledges updates

• Involves updates sent as required (when changes are seen) and every 30 minutes after no change has been seen

Differences BetweenRIPv1 and OSPF

RIPv1

• Can transmit info in two messages – routing update and triggered update

• Uses hop count as metric

OSPF

• Has protocols for discovering neighbors and forming adjacencies as well as protocols for sending updates through the network – these protocols alone add up to nine message types

• Uses cost as a metric – cost has the capacity to be a complex calculation; not defined in RFCs

Administrative Distance ValuesConnected interface or static route that IDs outgoing interfaceStatic routeEIGRP summary routeExternal BGP ??EIGRPIGRPOSPFRIPExternal EIGRPExternal BGP ??An unknown network

0

152090100110120170200255 or infinity

Static Routing Advantages

• Low processor overhead – routers don’t spend valuable CPU cycles calculating the best path. This requires less processing power and less memory (and, therefore, a less expensive router)

• No bandwidth utilization – routers don’t eat up bandwidth updating each other about static routes

Static Routing Advantages

• Secure operation – routers that don’t send updates won’t inadvertently advertise network information to an untrusted source. Routers that don’t accept routing updates are less vulnerable to attack

• Predictability – static routes enable an administrator to precisely control a router’s path selection. Dynamic routing often yields unexpected results, even in small networks

Static Routing Disadvantages

• High-maintenance configuration – administrators must configure all static routes manually. Complex networks may require constant reconfiguration

• No adaptability – statically configured routes can’t adapt to changes in link status

Dynamic Routing Advantages

• High degree of adaptability – routers can alert each other about downed links or newly discovered paths. Routers automatically “learn” a network’s topology and select optimum paths

• Low maintenance configuration – after the basic parameters for a routing protocol are set correctly, administrative intervention is not required

Dynamic Routing Disadvantages

• Increased processor overhead – dynamic routing processes can require a significant amount of CPU time and system memory

• High bandwidth utilization – routers must use bandwidth to send and receive routing updates, which can detrimentally affect performance on slow WAN links

Configuring Static Routing

• Using an Exit Interface– RTA (config)#ip route 10.6.0.0 255.255.0.0 s1

• Using a Next-Hop IP Address– RTA (config)#ip route 10.7.0.0 255.255.0.0 10.4.0.2

GlossaryAdjacent Neighbors

Area

Autonomous Switching

Router directly connected; must have same mask and hello parameters; can exchange routing information

A logical set of network segments and their attached devices, usually connected to other areas via routers making a single AS

Feature on Cisco router that provides faster packet processing by allowing ciscoBus to switch packets independently without interrupting the system processor

Glossary

Berkeley Standard Distribution

Cisco Express Forwarding

Term used to describe any of variety of UNIX-type OS based on UC Berkeley BSD operating system

Advanced layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns such as the Internet, on networks characterized by intensive web-based applications or interactive sessions

Glossary

Classful Routing Protocol

Classless Routing Protocol

Protocol that does not carry the subnet mask; usually distance vector; does not allow implementation of VLSM and summarization

Carries subnet mask in the routing update; allows implementation of VLSM and summarization

Glossary

Convergence

Count to Infinity

Speed and capability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology

Problem that can occur in routing algorithms that are slow to converge, in which routers continuously increment the hop count to particular networks. Typically, some hop count limit is imposed to prevent the problem

Glossary

DefaultRoutes

Diffusing Update Algorithm = DUAL

Route that should be used in the destination network is not present in the routing table

Convergence algorithm used in Enhanced IGRP that provides loop-free Operation at every instant throughout a route computation. This allows routers involved in a topology change to synchronize at the same time, while not involving routes that are unaffected by the change

Glossary

DijkstraAlgorithm

DistanceVectorProtocol

Iterates on length of path to determine a shortest path spanning tree. Used in LSA – sometimes called shortest path first algorithm

Iterates on number of hops in route to find shortest path spanning tree. Call for each router to send entire routing table in each update, but only to its neighbors. Can be prone to routing loops but computationally simpler than LSA. Uses Bellman Ford routing algorithm

GlossaryDynamicRoutes

ExteriorRouting

Fast Switching

Automatic rerouting of traffic based on sensing and analyzing current actual network conditions, not including cases of routing decisions taken on predefined info

Used to exchange info between AS or organizations, used to connect to Internet; BGP and EGP are examples

Cache in Cisco router that contains routing decisions. After routing decision for a packet made, it can be cached in any one of a variety of caches. Enhances forwarding of traffic

GlossaryFeasibleSuccessor

FlashUpdate

FloatingStaticRoute

Describes next hop router that has path to remote network that EIGRP considers a viable route

Sent asynchronously in response to change in network topology. If there is a change in metric, update sent immediately without waiting for timer to expire; aka triggered updates

Manually configured; identified as route to choose only if dynamically learned routes fail. Requires higher admin distance than routing protocol being used

Glossary

Flooding

IncrementalUpdate

Interior Routing Protocol

Traffic-passing technique used by switches and bridges to send out frame through all interfaces except port on which received

Sent only when there is a change in topology

Used to route information between routers in AS

Glossary

Link-stateAdvertisement

Link-state Algorithm

Neighbor

Broadcast packet used by link-state protocols that contains info about neighbors and path costs. Used by receiving routers to maintain routing tables

Each router broadcasts or multicasts info regarding the cost to reach each of its neighbors to all nodes in the network

In OSPF or EIGRP, two routers that have interfaces to a common network

GlossaryPoisonReverse

Redistri-bution

Routed Protocol

Routing updates that specifically indicate that a network or subnet is unreachable, rather than implying that a network is unreachable by not including it in updates

Allowing routing info discovered through one routing protocol to be distributed in the update messages of another routing protocol. Sometimes called route redistribution

Protocol that can be routed – AppleTalk, DECnet, IP, IPX

GlossaryRoutingFunction

RoutingLoop

RoutingProtocol

Process of finding a path to a destination host

Loop in which routing info is fed back to originating router; often happens when redistribution is configured; can lead to confusion if originating router loses the route – it may believe there is an alternate path

Accomplishes routing through the implementation of a specific algorithm – IGRP, OSPR, RIP

GlossarySilicon Switching

Split Horizon Rules

StaticRoute

Based on Silicon Switch Engine (SSE) which allows the processing of packets independent of the Silicon Switch Processor (SSP); provides high speed, dedicated packet switching

Routing technique in which info about routes is prevented from existing the router interface through which that info was received. Useful to prevent loops in small networks

Explicitly configured and entered into routing table

Glossary

SwitchingFunction

TopologyTable

TriggeredUpdate

Forwarding packets from an inbound interface to an outbound interface

Used by EIGRP and OSPF; table that records all the routes in the network before determining which will be entered into the routing table

Sent asynchronously in response to change in network topology. Same as flash update