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Knowledge Base

Delayed ISIS convergence when DWDM link fails

Publication Date:  2014-08-04  |   Views:  183  |   Downloads:  0  |   Author:  00724393  |   Document ID:  ETC0002802357


Issue Description

【Problem Summary】Delayed convergence when DWDM link fails
【Problem Details】Delayed convergence when DWDM link fails

There are 2 links in load balance (PLANE 1 and PLANE 2). ISIS as IGP and network is dual stacked. when link on PLANE 2 fails, traffic should switch to plane 1.
when this happen service is affected 3 minutes.

Handling Process

Analyse the logs and find that BFD for IPv4 detect failure as soon as physical link is down but the IPv6 ISIS will still be up.
Based on this we find IPv6 ISIS is still up even when the link is already down which lead to service being affected.

Root Cause

Because the network is dual stacked (IPv4+IPv6) and BFD for IPv4 ISIS is enabled but BFD for IPv6 ISIS is not enabled so if link goes down, traffic switchover will delay to switch to second link and affect service. Root cause is becuase the bfd is only for IPV4 and ISIS peer would not be down because IPv6 is still up.


【Resolution Summary】enable BFD for IPv6 ISIS
【Resolution Details】Solution Analysis:The transmission down happened again on 27th, but the BGP peer did not flap. The reason is ISIS peer expired only after 1 minutes for this time, but 2 minutes for the last time. If the hold time is 2 minutes, BGP would be down The bfd is only IPV4 and ISIS peer would not be down because IPv6 is still up. The IPv4 routes will still choose GE2/0/1 as the best path and the IPv4 peer is absent on this interface, so the related IPv4 routes dispear. Resolution Detail: Suggest to enable BFD for IPv6 ISIS // Risk of Solution:Risky Operation


enable BFD for IPv6 ISIS