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| 5.5 |  | Troubleshooting IGRP |
| 5.5.1 |  | Discontiguous networks |

When a major network is separated by another major network, this is called a discontiguous network. Figure shows an example of a discontiguous network in which 137.99.0.0 is a major network. The subnets of this major network are separated by another major network, 131.108.0.0. This situation produces a discontiguous network problem.

Debugs and Verification
Figure shows the configuration of routers R1 and R2. IGRP is enabled on the Ethernet interfaces of routers R1 and R2 with the correct network statements.

Figure shows the debug ip igrp transactions command output for router R1 and R2. The debugs show that both routers are sending to each other the summarized major network address for 137.99.0.0, instead of their specific network addresses. As a result, both routers will ignore the less specific 137.99.0.0 update because they are already connected to this major network.

Solution
IGRP does not support discontiguous networks. Several solutions to this problem exist. One solution is to configure a static route on each router to the specific 137.99.0.0 subnet on the other router as shown in Figure . Since IGRP is a classful routing protocol and does not include the subnet information in its routing updates, configuring the static routes is a “patch” to fix this problem.

Another solution is to replace the classful IGRP routing protocol with a classless routing protocol, such as RIP v2, OSPF, EIGRP, or IS-IS. Unlike RIP, there is not a version of IGRP that is a classless routing protocol.

If using IGRP is a must, there are two solutions. The first solution is to change the address on the link between routers R1 and R2 to be part of the 137.99.0.0 network. In other words, assign another subnet on this link, which is part of 137.99.0.0.

If the addresses cannot be changed, a Generic Routing Encapsulation (GRE) tunnel can be configured between routers R1 and R2. A separate subnet address with the same mask is configured on the tunnel interface, as demonstrated.

Discontiguous networks are discouraged and should be avoided even if a protocol supports it, as they limit the summarization capabilities. Figure shows the routing table entry for R2 after fixing this problem.

> Note: Since IGRP is a classful routing protocol, it does not carry the subnet mask in its updates. Thus, the same issue with discontiguous subnets can apply when using VLSM with IGRP. The only solution is to change the subnet mask so that VLSM is not used. The long-term solution should be to change to EIGRP or another classless routing protocol.