© 2002, Cisco Systems, Inc. All rights reserved. 1 Determining IP Routes.

© 2002, Cisco Systems, Inc. All rights reserved. 1

Determining IP Routes

© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-2

Routed Versus Routing Protocol

• A routed protocol: – Includes any network protocol suite that provides

enough information in its network layer address to allow a router to forward it to the next device and ultimately to its destination.

– Defines the format and use of the fields within a packet.

• A routing protocol: – Provides processes for sharing route information.

– Allows routers to communicate with other routers to update and maintain the routing tables.


IP as a Routed Protocol

• IP is a connectionless, unreliable, best-effort delivery protocol.

• As information flows down the layers of the OSI model; the data is processed at each layer.

• IP accepts whatever data is passed down to it from the upper layers.


Packet Propagation and Switching Within a Router


Packet Propagation and Switching Within a Router

• As a frame is received at a router interface. • The MAC address is checked to see if the

frame is directly addressed to the router interface, or a broadcast.

• The frame header and trailer are removed and the packet is passed up to Layer 3.

• The destination IP address is compared to the routing table to find a match.

• The packet is switched to the outgoing interface and given the proper frame header.

• The frame is then transmitted.


To route, a router needs to do the following:• Know the destination address

• Identify the sources it can learn from

• Discover possible routes

• Select the best route

• Maintain and verify routing information

What Is Routing?


• Routers must learn destinations that are not directly connected.

What Is Routing? (Cont.)


Static Route• Uses a route that a

network administrator enters into the router manually

Dynamic Route• Uses a route that a

network routing protocol adjusts automatically for topology or traffic changes

Identifying Static and Dynamic Routes


Static Routes

• Configure unidirectional static routes to and from a stub network to allow communications to occur.


Static Route Example

• This is a unidirectional route. You must have a route configured in the opposite direction.


Default Routes

• This route allows the stub network to reach all known networks beyond router A.


Verifying the Static Route Configuration

router#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 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 default U - per-user static route Gateway of last resort is 0.0.0.0 to network 0.0.0.0 10.0.0.0/8 is subnetted, 1 subnetsC 10.1.1.0 is directly connected, Serial0S* 0.0.0.0/0 is directly connected, Serial0


• Routing protocols are used between routers to determine paths and maintain routing tables.

• Once the path is determined, a router can route a routed protocol.

What Is a Routing Protocol?


Routers discover the best path to destinations from each neighbor.

Sources of Information and Discovering Routes


Classes of Routing Protocols


Distance Vector• RIPv1

• RIPv2

Link State• OSPF

• IS-IS

Hybrid Routing• EIGRP

Classes of Routing Protocols


Distance Vector Routing Protocols


• Routers pass periodic copies of routing table to neighbor routers and accumulate distance vectors.

Distance Vector Routing Protocols


• Maximum is 6 paths (default = 4)

• Hop-count metric selects the path

• Routes update every 30 seconds

RIP (Distance Vector Routing Protocol)


• Starts the RIP routing process

Router(config)#router rip

Router(config-router)#network network-number

• Selects participating attached networks

• Requires a major classful network number

RIP Configuration


RIP Configuration Example


Displaying the IP Routing Table


RIPv2

• Distance vector protocol like RIP.

• Same AD, update timer and metric as RIP.

• But supports classless ip routing.

• Uses multicast address 224.0.0.9

© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-24© 2002, Cisco Systems, Inc. All rights reserved. 24

Link-State Routing(OSPF)


• After initial flood, pass small event-triggered link-state updates to all other routers

Link-State Routing Protocols


Benefits of Link-State Routing

• Fast convergence: changes are reported immediately by the source affected.


OSPF as a Link-State Protocol

• OSPF propagates link-state advertisements rather than routing table updates.

• LSAs are flooded to all OSPF routers in the area.

• The OSPF link-state database is pieced together from the LSAs generated by the OSPF routers.

• OSPF uses the SPF algorithm to calculate the shortest path to a destination.

–Link = router interface

–State = description of an interface and its relationship to neighboring routers


OSPF Hierarchical Routing

• Consists of areas and autonomous systems

• Minimizes routing update traffic


Configuring Single Area OSPF

Router(config-router)#network address mask area area-id

• Assigns networks to a specific OSPF area

Router(config)#router ospf process-id

• Defines OSPF as the IP routing protocol


OSPF Configuration Example


Router#show ip ospf interface

Verifying the OSPF Configuration

• Displays area-ID and adjacency information

Router#show ip protocols

• Verifies that OSPF is configured

Router#show ip route

• Displays all the routes learned by the router

Router#show ip ospf neighbor

• Displays OSPF-neighbor information on a per-interface basis


Drawbacks to Link-State Routing Protocols

• Initial discovery may cause flooding.

• Memory- and processor-intensive.

© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-33© 2002, Cisco Systems, Inc. All rights reserved. 33

Balanced Hybrid Routing( EIGRP)


• Shares attributes of both distance vector and link-state routing

Balanced Hybrid Routing


Introducing EIGRP

EIGRP FEATURES• Metric: Bandwidth, delay, load, reliability

• Event triggered updates

• Exchange updates like distance vector.


EIGRP Terminology


Configuring EIGRP

Router(config-router)#network network-number

• Selects participating attached networks

Router(config)#router eigrp autonomous-system

• Defines EIGRP as the IP routing protocol


EIGRP Configuration Example


Verifying the EIGRP Configuration

Router#show ip protocols

Router#show ip route eigrp

Router#show ip eigrp traffic

Router#show ip eigrp neighbors

Router#show ip eigrp topology

• Displays the neighbors discovered by IP EIGRP

• Displays the IP EIGRP topology table

• Displays the number of IP EIGRP packets sent and received

• Displays current EIGRP entries in the routing table

• Displays the parameters and current state of the active routing protocol process

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© 2002, Cisco Systems, Inc. All rights reserved. 1 Determining IP Routes.

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