Question

A service provider deploys an MPLS TE tunnel across a core where every physical link supports a 1500-byte IP MTU. Customers send 1500-byte IP packets with the DF bit set. After the TE tunnel is activated, users report that large TCP transfers fail while small pings succeed. What is the most likely cause and correct remedy?

Accepted Answer

The MPLS label stack adds overhead that pushes labeled packets beyond the core link MTU, causing drops of DF-set packets; increase the core interface MPLS MTU to accommodate the label stack (e.g., 1500 + label bytes).

Answer

TE tunnels cannot carry packets larger than 576 bytes; enable IP fragmentation on the tunnel headend to split traffic.

Answer

The DF bit forces the tunnel into pipe mode; clear the DF bit on the PE to allow the core to fragment the packets.

Answer

RSVP-TE signaling reserves MTU bandwidth incorrectly; lower the tunnel bandwidth reservation so smaller packets are prioritized.