Is there a difference between neighborship and adjacency in case os IS-IS? Many times I come across both the words in the context of IS-IS. Do they have different meanings based on the IS-IS states?


1 Answer 1


From the RFC1142 OSI IS-IS Intra-domain Routing Protocol

3.6 Additional Definitions ...

3.6.2 Neighbour: An adjacent system reachable by tra versal of a single subnetwork by a PDU.

3.6.3 Adjacency: A portion of the local routeing information which pertains to the reachability of a single neighbour ES or IS over a single circuit. Adjacencies are used as input to the Decision Process for forming paths through the routeing domain. A separate adjacency is created for each neighbour on a circuit, and for each level of routeing (i.e. level 1 and level 2) on a broadcast circuit.

Strictly speaking as per RFC, proper wording is adjacency.

Personally unless there is some other connotation or background context referenced, you will hear as industry slang the use of both with no distinction, ie:

"Is the adjacency up", "Hey, did neighborship restore?"


@monica as far as OSPF, I think about it as in the finite state machine, meaning ''2-way'' is bidirectional communication where the router id has been received in the hello packet and "Full" database fully synchronized.

As per RFC2328:


A relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers become adjacent.

The word neighborship is not stated in the definitions of commonly used terms.

  • Thanks for RFC wordings. So in case of ISIS, is it not strictly like OSPF where neighborship means "seeing own address in neighbor's hello" and adjacency means "databases fully synchronised"?
    – monica
    Commented Feb 21, 2018 at 6:16
  • @monica Please share your source for this information.
    – DRP
    Commented Feb 21, 2018 at 6:29

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