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Cisco Bug: CSCtd05126 - mpls mtu is passed to icmp without the mpls stack adjustment

Last Modified

May 16, 2018

Products (1)

  • Cisco Carrier Routing System

Known Affected Releases

3.8.1.BASE

Description (partial)

Symptom:
When ip datagram carried in mpls packet has DF bit set and the packet needs to be fragmented because
the pkt size is greater than the interface mtu, the packet is dropped and a ICMP unreacchable with
need fragmentation icmp code is sent back to the source of the packet. The issue is that mpls mtu
value in the ICMP message does not account for the mpls label stack on the pkt. Hence, the mtu value
sent is larger than the correct value. This can result in tcp session flap which could cause e.g. BGP 
session flap.


Conditions:

When ip datagram carried in mpls packet has DF bit set and the packet needs to be fragmented because
the pkt size is greater than the interface mtu.
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  • Full Description (including symptoms, conditions and workarounds)
  • Status
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  • Known Fixed Releases
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