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| 5.8 |  | Troubleshooting IS-IS |
| 5.8.1 |  | IS-IS adjacency problems |

IS-IS adjacency-related problems normally are caused by link failures and configuration errors. On Cisco routers, inspecting the output of the show interface command can easily identify link failures. Also, because IS-IS routing is not required to establish IP connectivity to directly attached routers, it is easy to discern whether the problem is media-related or specific to the IS-IS configuration.

The show clns neighbors command is usually the starting point for troubleshooting IS-IS adjacency problems. The output of this command should list all neighbors expected to be adjacent to the router being investigated. The command show clns is-neighbors provides similar output, but it is intended to list only neighbor routers or IS-IS adjacencies; whereas show clns neighbors lists all types of adjacencies, both for IS-IS and ES-IS.

Using the output from the show clns neighbor and show clns neighbor detail commands, IS-IS adjacencies can be examined. Problems with IS-IS adjacency formation can be registered by the presence of fewer neighbors than expected or a situation in which the status of an expected adjacency is not up. Another symptom could be that the neighbor is known through ES-IS protocol instead of IS-IS.

The router RT1 has properly formed adjacencies with its directly connected neighbors, RT2 and RT5. All ISO devices run the ES-IS protocol to facilitate mutual discovery and communication between end systems and routers in the CLNS environment. End systems and routers exchange end-system hellos (ESHs) and intermediate-system hellos (ISHs) within the ES-IS framework. Connected routers also receive each other’s ISHs and form ES-IS adjacencies. Therefore, it is possible that ES-IS adjacencies might still be formed between two routers even if there are problems with the IS-IS adjacency.

Figure also shows output from show clns neighbors, but shows problems with the adjacencies formed with RT2 and RT5. In this example, the IS-IS adjacency with RT2 is in INIT state instead of UP. The protocol is correctly shown as IS-IS. The adjacency with RT5 shows UP, however the protocol is ES-IS instead of IS-IS. As explained previously, the ES-IS protocol runs independently of IS-IS; therefore, the ES-IS adjacency formed between RT1 and RT5 has nothing to do with IS-IS.

These routers cannot form an IS-IS adjacency with each other, apparently because of a problem in the configuration or the IS-IS environment. Most adjacency problems related to the IS-IS environment can be debugged with the debug isis adj-packets command. The output of this command can be daunting if the router under inspection has a lot of neighbors because the display shows all the hellos transmitted and received by the local routers.