Technical Tip: IS-IS neighbors stuck in INIT state when there are more than 42 neighbors

[jo_rang]

Description	This article describes a solution to the issue of IS-IS neighbors being stuck in the INIT status on FortiGate devices when there are more than 42 neighbors configured.
Scope	FortiOS v7.0, v7.2.
Solution	In a network environment where there are more than 42 IS-IS-enabled routers, FortiGates running versions 7.0 or 7.4 may display in the neighbor table some of the neighbors in an INIT state.  get router info isis neighbor newtownfg01 # get router info isis neighbor System Id Interface SNPA State Holdtime Type Protocol 0000.0000.0004 port4 xxxx.xxxx.3280 Up 9 L2 IS-IS 0000.0000.0006 port4 xxxx.xxxx.5220 Up 9 L2 IS-IS 0000.0000.0005 port4 xxxx.xxxx.4b23 Init 22 L2 IS-IS   <-- Some of the neighbors will be stuck in the INIT state. 0000.0000.0008 port4 xxxx.xxxx.3c79 Init 22 L2 IS-IS 0000.0000.0012 port4 xxxx.xxxx.4dc1 Init 22 L2 IS-IS 0000.0000.0013 port4 xxxx.xxxx.a187 Init 22 L2 IS-IS This issue is caused because in the ISIS Hello packet, the FortiGate only includes one TVL field for neighbors. The field size is 252 bytes, which is only enough for 42 neighbors. To confirm this, this is the reason the neighbor is in an INIT state, perform a packet capture with Advanced Filter: The ISIS Hello Packet will show only on the TVL field for Neighbors.  The solution is to upgrade to version 7.4.0 or later, where improvements were made to add the TVL field when the number of neighbors exceeds 42. The Following screenshot shows the FortiGate (running version 7.4.8) adding the TVL field to include more neighbors Related documents: Technical Tip : FortiGate IS-IS configura... - Fortinet Community diagnose ip router isis