The Enhanced Interior Gateway Routing Protocol (EIGRP), referred to as an advanced Distance Vector protocol, offers radical improvements over IGRP. Traditional DV protocols such as RIP and IGRP exchange periodic routing updates with all their neighbors, saving the best distance (or metric) and the vector (or next hop) for each destination. EIGRP differs in that it saves not only the best (least-cost) route but all routes, allowing convergence to be much quicker. Further, EIGRP updates are sent only upon a network topology change; updates are not periodic.
Getting EIGRP running is not much more difficult than getting IGRP running, as we will see in Section 4.1.
Even though EIGRP offers radical improvements over IGRP, there are similarities between the protocols. Like IGRP, EIGRP bases its metric on bandwidth, delay, reliability, load, and MTU (see Section 4.2).
The fast convergence feature in EIGRP is due to the Diffusing Update Algorithm (DUAL), discussed in Section 4.3.
EIGRP updates carry subnet mask information. This allows EIGRP to summarize routes on arbitrary bit boundaries, support classless route lookups, and allow the support of Variable Length Subnet Masks (VLSM). This is discussed in Section 4.4 and Section 4.5.
Setting up default routes in EIGRP is discussed in Section 4.6.
Troubleshooting EIGRP can be tricky. This chapter ends with some troubleshooting tips in Section 4.7.
This chapter focuses on EIGRP’s enhancements over IGRP: the use of DUAL; and the use of subnet masks in updates, which in turn allow VLSM and route summarization at arbitrary bit boundaries. This chapter does not cover router metrics in detail or the concept of parallel paths. Those concepts have not changed much in EIGRP. I assume that the reader is familiar with IGRP.
TraderMary’s network, shown in Figure 4-1, can be configured to run EIGRP as follows.
Just like RIP and IGRP, EIGRP is a distributed protocol that needs to be configured on every router in the network:
hostname NewYork ... interface Ethernet0 ip address 172.16.1.1 255.255.255.0 ! interface Ethernet1 ip address 192.168.1.1 255.255.255.0 ! interface Serial0 description New York to Chicago link ip address 172.16.250.1 255.255.255.0 ! interface Serial1 description New York to Ames link bandwidth 56 ip address 172.16.251.1 255.255.255.0 ...
router eigrp 10
network 172.16.0.0hostname Chicago ... interface Ethernet0 ip address 172.16.50.1 255.255.255.0 ! interface Serial0 description Chicago to New York link ip address 172.16.250.2 255.255.255.0 ! interface Serial1 description Chicago to Ames link ip address 172.16.252.1 255.255.255.0 ...
router eigrp 10
network 172.16.0.0hostname Ames ... interface Ethernet0 ip address 172.16.100.1 255.255.255.0 ! interface Serial0 description Ames to Chicago link ip address 172.16.252.2 255.255.255.0 ! interface Serial1 description Ames to New York link bandwidth 56 ip address 172.16.251.2 255.255.255.0 ...
router eigrp 10
The syntax of the EIGRP command is:
in global configuration mode. The networks that will be participating in the EIGRP process are then listed:
What does it mean to list the network numbers participating in EIGRP?
Router NewYork will include directly connected
172.16.0.0subnets in its updates to neighboring routers. For example,
172.16.1.0will now be included in updates to the routers Chicago and Ames.
NewYork will receive and process EIGRP updates on its
172.16.0.0interfaces from other routers running EIGRP 10. For example, NewYork will receive EIGRP updates from Chicago and Ames.
By exclusion, network
192.168.1.0, connected to NewYork, will not be advertised to Chicago or Ames, and NewYork will not process any EIGRP updates received on Ethernet0 (if there is another router on that segment).
The routing tables for NewYork,
Chicago, and Ames will show
172.16.0.0 subnets. Here is
NewYork#sh ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP 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 default Gateway of last resort is not set 1
172.16.0.0/24 is subnetted, 6 subnetsD 172.16.252.0 [90/2681856] via 172.16.250.2, 00:18:54, Ethernet0/0 C 172.16.250.0 is directly connected, Ethernet0/0 C 172.16.251.0 is directly connected, Ethernet0/1 D 172.16.50.0 [90/2195456] via 172.16.250.2, 00:18:54, Ethernet0/0 C 172.16.1.0 is directly connected, Loopback0 D 172.16.100.0 [90/2707456] via 172.16.250.2, 00:18:54, Ethernet0/0 C 192.168.1.0/24 is directly connected, Loopback1
The EIGRP-derived routes in this table are labeled with a
“D” in the left margin. Note that
the routing table provides summary information (as in line 1). Line 1
contains subnet mask information (24 bits, or
255.255.255.0) and the number of subnets in
NewYork#sh ip eigrp topology IP-EIGRP Topology Table for process 10 Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - Reply status P 172.16.252.0/24, 1 successors, FD is 2681856 via 172.16.250.2 (2681856/2169856), Serial0 via 172.16.251.2 (46738176/2169856), Serial1 P 172.16.250.0/24, 1 successors, FD is 2169856 via Connected, Serial0 P 172.16.251.0/24, 1 successors, FD is 46226176 via Connected, Serial1 P 172.16.50.0/24, 1 successors, FD is 2195456 via 172.16.250.2 (2195456/281600), Serial0 P 172.16.1.0/24, 1 successors, FD is 128256 via Connected, Ethernet0 2
P 172.16.100.0/24, 1 successors, FD is 27074563
via 172.16.250.2 (2707456/2195456), Serial04
via 172.16.251.2 (46251776/281600), Serial1
This topology table shows two entries for
172.16.100.0 (line 2). Only the lower-cost route
(line 3) is installed in the routing table, but the second entry in
the topology table (line 4) allows NewYork to
quickly converge on the less preferred path if the primary path
Note that network
192.168.1.0, defined on
Ethernet1, did not appear in the routing tables
of Chicago and Ames. To be
192.168.1.0 would have to be defined
in a network statement under the EIGRP configuration on
hostname NewYork ... router eigrp 10 network 172.16.0.0 network 192.168.1.0
Each EIGRP process is identified by an autonomous system (AS) number, just like IGRP processes. Routers with the same AS numbers will exchange routing information with each other, resulting in a routing domain . Routers with dissimilar AS numbers will not exchange any routing information by default. However, routes from one routing domain can be leaked into another domain through the redistribution commands -- this is covered in Chapter 8.
Compare the routing table in this section with the corresponding table for IGRP in Chapter 3. The essential contents are identical: the same routes with the same next hops. However, the route metrics look much bigger and the route update times are very high. IGRP routes would have timed out a while ago.
EIGRP metrics are essentially derived from IGRP metrics. The following section provides a quick summary.