R2511-HP MSR Router Series MPLS Configuration Guide(V5)

• An interface always forwards MPLS packets carrying L2VPN packets, even if the MPLS packet size

exceeds the interface MPLS MTU. However, whether the forwarding succeeds depends on the

actual forwarding capacity of the interface.

• An interface drops MPLS packets carrying IPv6 packets if the MPLS packet size exceeds the

interface MPLS MTU. At the same time, the device sends the interface MPLS MTU to the sender in

a "Packet Too Big" ICMPv6 message.

• If the interface MPLS MTU is greater than the interface MTU, data forwarding might fail on the

interface. If no MPLS MTU is configured, MPLS packet fragmentation is based on the interface MTU.

Because the length of fragments does not take the MPLS labels into account, an MPLS fragment

might exceed the interface's MTU.

Configuring a TTL processing mode for an LSR

An LSR can process the TTL field in the either of the following ways:

• Enable TTL propagation—When an LSR labels a packet, it copies the TTL value of the original

packet (the IP TTL value of an IP packet or the TTL value in the top label of a labeled packet) to the

TTL field of the newly added label. When an LSR pops the stack-top label, it copies the TTL value of

the stack-top label to the TTL field of the original packet. In this mode, the TTL value of a packet is

decreased hop by hop when forwarded along the LSP. The tracert result reflects the real path along

which the packet has traveled.

Figure 10 TTL processing when TTL propagation is enabled

• Disable TTL propagation—When an LSR labels a packet, it does not copy the TTL value of the

original IP packet to the TTL field of the label, and the label's TTL is set to 255. When an LSR pops

the stack-top label, it does not copy the label's TTL to the original packet, and if the LSR is the egress

LSR, it decreases the TTL value of the original packet by 1. Other LSRs do not change the TTL value

of the original packet. In this mode, the tracert result does not show the hops within the MPLS

backbone, as if the ingress and egress were connected directly.