Cisco Bug: CSCtd05126 - mpls mtu is passed to icmp without the mpls stack adjustment
May 16, 2018
- Cisco Carrier Routing System
Known Affected Releases
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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