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Cisco Bug: CSCvt72736 - DF-Bit Not Preserved In Outer IPv4 Header When Using MPLS And Tunnel PMTUD On Tunnel Interface

Last Modified

Sep 18, 2020

Products (1)

  • Cisco ASR 1000 Series Aggregation Services Routers

Known Affected Releases

15.0(0.4)S

Description (partial)

Symptom:
When "tunnel path-mtu-discovery" and "mpls ip" are configured on a tunnel interface, it breaks expected behavior of the "tunnel path-mtu-discovery" command for traffic in MPLS VPNv4 learned VRF subnets.  In a working state, this command should preserve the state of the df-bit in the original data packet and set that same value for the df-bit in the outer tunnel IPv4 header.  When these specific conditions are met, if the df-bit is set in the original data packet it is not carried over/set in the outer tunnel IPv4 header as expected.

Conditions:
1. Configure tunnel interface with "mpls ip" and "tunnel path-mtu-discovery" commands
2. Share VPNv4 VRF subnets via MPLS/BGP over the tunnel
3. Have PMTUD enabled host in a VPNv4 learned subnet send traffic with df-bit set
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