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Cisco Bug: CSCvi05697 - NCS5500 traceroute issue due to incorrect ttl on PHP

Last Modified

Oct 20, 2019

Products (1)

  • Cisco Network Convergence System 5500 Series

Known Affected Releases

6.2.3.BASE

Description (partial)

Symptom:
Topology:
CE (Cisco) >> PE (ASR9K) >> P (NCS5500) >>  PE (ASR9K) >> Public Internet(multiple hops) >> Final Destination

With "mpls ip-ttl-propagate disable" on Ingress PE, the trace route  from CE does not show any hops outside mpls core (i.e. Public Internet) and goes through to the destination directly. This happens only for Fretta boxes acting as PHP router. The problem is when NCS is doing PHP and copying the mpls ttl back to ip ttl in ip packet, that is not done correctly and hence we are copying mpls ttl of 255 back to ip tll.

This is confirmed by packet capture on egress PE showing incorrect TTL vaule for the traceroute packets.

With "no mpls ip-ttl-propagate disable" i .e. TTL propagation enabled, you would not see this problem.

Conditions:
NCS5500 acting as PHP and "mpls ip-ttl-propagate disable" on Ingress PE
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