R3303-HP HSR6800 Routers MPLS Configuration Guide
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• If fragmentation is allowed, the LSR removes the label stack from the packet, fragments the IP packet
(the length of a fragment is the MPLS MTU minus the length of the label stack), adds the label stack
back into each fragment, and then forwards the fragments.
• If fragmentation is not allowed, the LSR drops the packet directly.
To configure the MPLS MTU of an interface:
Ste
p
Command
Remarks
1. Enter system view.
system-view N/A
2. Enter interface view.
interface interface-type
interface-number
N/A
3. Configure the MPLS MTU of
the interface.
mpls mtu value
By default, the MPLS MTU of an
interface is not configured.
The following applies when an interface handles MPLS packets:
• 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.