IS-IS neighbor relationship is maintained by Hello packets. IS-IS Hello packets are filled in by MTUs. If the MTU increases, the length of IS-IS Hello packets increases. The transmission device between IS-IS neighbor relationship may fail to transparently transmit IS-IS Hello packets and discard the packets. As a result, IS-IS neighbor goes Down.

The problem occurs because the remote peer fails to receive IS-IS Hello packets. Run the ping -s packetsize host command to check information about the transmission link.

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