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IS-IS adjacency is not formed due to MTU mismatch

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.


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.

New Member

How about only changing the clns mtu ONLY so that they match up, leave the IP mtu to be different.

That works like a charm for ISIS neighborship to come up

R1 s2/0 -- R2 s2/0


int s2/0

mtu 17001

clns mtu 9216


int s2/0

mtu 17008

clns mtu 9216