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| 5.8 |  | Troubleshooting IS-IS |
| 5.8.4 |  | ES-IS adjacency instead of IS-IS adjacency formed |

Cisco routers running IS-IS in IP environments still listen to ISHs, intermediate-system hellos, generated by ES-IS protocol, in conformance with ISO 10589 requirements. When the physical and data-link layers are operational, an ES-IS adjacency can be formed even if appropriate conditions do not exist for establishing an IS-IS adjacency.

Figure shows what the output for the show clns neighbors command looks like when an ES-IS adjacency is formed instead of an IS-IS adjacency. This is usually because IS-IS hellos are not being processed, as a result of interface MTU mismatch or misconfigured authentication.