Core Issue
Integrated Intermediate System-to-Intermediate System (IS-IS) is a link-state routing protocol that has evolved from ISO IS-IS. Integrated IS-IS supports Connectionless Network Service (CLNS) only, IP only or dual (both CLNS and IP) environments.
IS-IS routers that share the same data link layer form adjacencies and are able to exchange routing information. An adjacency is created for each level of routing, one for Level 1 and another for Level 2. IS-IS hello packets are used to discover and maintain adjacencies. By default, the hello packets are padded to the full Maximum Transmission Unit (MTU) size. IS-IS adjacency formation may fail due to MTU mismatch on a link.
Resolution
To solve adjacency formation problems caused by an MTU mismatch, configure the same MTU value on the devices on the link.
To adjust the maximum packet size, issue the mtu command in interface configuration mode.
For more information, refer to MTU Mismatch Problem in IS-IS.
