EIGRP Behavior

Unlike IGRP, EIGRP is an advanced distance vector protocol that carries the subnet mask information when an update is sent out. Therefore, EIGRP supports discontiguous network and variable-length subnet masking (VLSM). For more explanation about discontiguous networks and VLSM, refer to Chapter 2, "Understanding Routing Information Protocol (RIP)." Figure 6-8 shows the network diagram that illustrates EIGRP's support for discontiguous networks.

Figure 6-8. Example of EIGRP Support for Discontiguous Networks

Figure 6-8. Example of EIGRP Support for Discontiguous Networks

Figure 6-8 shows two routers connected through a serial port. Router B has the network 192.168.8.128/25 that needs to advertise to Router A across the network 10.1.1.0/24. By default, EIGRP is a classful routing protocol; Router B will autosummarize the route across the major network boundary. Therefore, Router B will advertise 192.168.8.0/24 to Router A, which will ignore this route advertisement. To make EIGRP support discontiguous networks, you must configure the no auto-summary command under the command router eigrp. With the no auto-summary command in place in Router B, Router B will advertise the 192.168.8.128/25 route to Router A, and Router A will have a routing entry for the route. The problem with discontiguous network then will be solved.

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