Sunday, September 27, 2009

IP Routing - EIGRP

Right, first a bit about EIGRP from my notes.

EIGRP (Enhanced Interior Gateway Routing Protocol) a cisco proprietry hybrid routing protocol and uses the DUAL algorithm for selecting the best path to remote networks. It has both link state and distance vector characteristics. EIGRP has an administrative distance of 90.

EIGRP uses a multicast address of 224.0.0 to send updates as the topology changes. If the router does not receive a reply from a neighbor after sending the multicast update it will use unicast. A list of neighbors are maintained in the neighborship table. After the router has sent 16 unicasts and recieved no reply the neighbor will be declared dead and removed from the neighborship table.

EIGRP will only share routing information with it's neighbors if they share the same AS number. All updates that EIGRP receives are entered into it's topology table and the best routes are selected by DUAL and entered into the routing table.

EIGRP (unlike IGRP) includes the subnet mask in it's advertisements which allows it to utilise VLSM and summarisation and supports discontiguous networks. By default EIGRP uses bandwidth and delay to calculate the best route to a remote network. EIGRP can loadbalance across up to 6 equal or unequal cost links but the default is 4.

My Lab

I'll be using the same lab as in previous posts.

Router 1 (R1)
FastEthernet 0/0 -
loopback 0 -
loopback 1 -

Router 2 (R2)
FastEthernet 0/0 -
FastEthernet 1/0 -

Router 3 (R3)
FastEthernet 0/0 -

Configuring EIGRP

Below I'm going to remove OSPF which was set up in a previous lab and configure all of my routers with EIGRP.

R1#sh ip protocols
Routing Protocol is "ospf 10"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID
It is an autonomous system boundary router
Redistributing External Routes from,
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks: area 0 area 0
Reference bandwidth unit is 100 mbps
Routing Information Sources:
Gateway Distance Last Update 110 00:01:01
Distance: (default is 110)

First I'll remove OSPF

R1#conf t
R1(config)#no router ospf 10

And now I'll configure EIGRP with an AS of 10. To configure EIGRP on all the routers in the diagram I use the following commands:

R1#conf t
R1(config)#router eigrp 10

R2#conf t
R2(config)#no router ospf 10
R2(config)#router eigrp 10

R3#conf t
R3(config)#no router ospf 10
R3(config)#router eigrp 10

I now check my routing table on R3 to make sure I see the routes from R1.

R3#sh ip route
Gateway of last resort is not set
D [90/158720] via, 00:00:14, FastEthernet0/0
D [90/30720] via, 00:00:14, FastEthernet0/0
C is directly connected, FastEthernet0/0

Routes starting with a D are EIGRP routes. Now I check I can ping one of the remote networks.

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/29/52 ms

I also need to set the default network so it gets advertised through EIGRP.

R1(config)#ip default-network

Now supposing I want to prevent one of the interfaces on a router from sending out or receiving advertisements.

R2(config)#router eigrp 10
R2(config-router)#passive-interface fastEthernet 1/0

The following message is displayed on the console screen to indicate that the interface will not be sending out EIGRP routes.

00:52:21: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 10: Neighbor (FastEthernet1/0) is down: interface passive

And on R3 I get the following message:

00:52:27: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 10: Neighbor (FastEthernet0/0) is down: Interface Goodbye received

On checking R3's routimg table I see that all the EIGRP routes are dropped.

R3#sh ip route
C is directly connected, FastEthernet0/0

Troubleshooting and Debug Commands

R3#sh ip route summary

R1#sh ip protocols

R2#sh ip eigrp interfaces
R3#debug ip eigrp
R2#sh ip eigrp traffic 10
R2#sh ip eigrp neighbors detail

No comments: