A Device Cannot Learn IS-IS Routes from Its Neighbor

Procedure

1. Check whether IS-IS neighbor relationship has been established between the device and its neighbor.

   Run the display isis peer command on each device on the link to check whether IS-IS neighbor relationship has been established.

   If IS-IS neighbor relationship is not established, rectify the fault according to Failed to Establish IS-IS Neighbor Relationships.

2. Check whether the IS-IS routing table of the device is correct.

   Run the display isis route command on the device to check the IS-IS routing table.

   1. If the IS-IS routing table contains specified routes, run the display ip routing-table ip-address [ mask | mask-length ] verbose command to check whether the IP routing table contains routes with higher protocol preference than IS-IS routes.

      

      If the State field of a route displays Active Adv, the route is active. If there are routes that have the same prefix but are discovered by different routing protocols, routes with higher protocol preference are preferred as active routes.

   2. If the IP routing table contains routes with higher protocol preference than IS-IS routes, modify the configuration based on network planning.

3. Check whether the device and its neighbor have the matching IS-IS cost style.

   Run the display current-configuration configuration isis command on the device and its neighbor to check the IS-IS cost style.

   The device can learn IS-IS routes from its neighbor when it has the same IS-IS cost style as its neighbor.

   The IS-IS cost style of a device can be set as follows:

   • narrow: indicates that the device can receive and send packets with cost style narrow.

   • narrow-compatible: indicates that the device can receive packets with cost style narrow or wide but sends only packets with cost style narrow.

   • compatible: indicates that the device can receive and send packets with cost style narrow or wide.

   • wide-compatible: indicates that the device can receive packets with cost style narrow or wide but sends only packets with cost style wide.

   • wide: indicates that the device can receive and send packets with cost style wide.

   If the IS-IS cost styles of both ends are set to narrow and wide (or wide-compatible) respectively, the two ends cannot communicate.

   If the IS-IS cost styles of both ends are set to narrow-compatible and wide respectively, the two ends cannot communicate either.

   If the device and its neighbor have mismatching IS-IS cost styles, run the cost-style command on the device to modify the configuration.