HP MSR2000/3000/4000 Router Series MPLS Configuration Guide

6
Configuring MPLS MTU
MPLS inserts the label stack between the link layer header and network layer header of each packet. To
make sure the size of MPLS labeled packets is smaller than the MTU of an interface, configure an MPLS
MTU on the interface.
MPLS compares each MPLS packet against the interface MPLS MTU. When the packet exceeds the MPLS
MTU:
• If fragmentation is allowed, MPLS 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
to each fragment, and forwards the fragments.
• If fragmentation is not allowed, the LSR drops the packet.
To configure an MPLS MTU for 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 an MPLS MTU for
the interface.
mpls mtu value
By default, no MPLS MTU is
configured on an interface.
The following applies when an interface handles MPLS packets:
• MPLS packets carrying IPv6 packets are always forwarded by an interface, even if the length of the
MPLS packets exceeds the MPLS MTU of the interface. Whether the forwarding can succeed
depends on the actual forwarding capacity of the interface.
• If the MPLS MTU of an interface is greater than the MTU of the interface, data forwarding might fail
on the interface.
• If you do not configure the MPLS MTU of an interface, fragmentation of MPLS packets is based on
the MTU of the interface without considering MPLS labels. An MPLS fragment might be larger than
the interface MTU and be dropped.
Specifying the label type advertised by the egress
In an MPLS network, an egress can advertise the following types of labels:
• Implicit null label with a value of 3.
• Explicit null label with a value of 0.
• Non-null label.
For LSPs established by a label distribution protocol, the label advertised by the egress determines how
the penultimate hop processes a labeled packet.
• If the egress advertises an implicit null label, the penultimate hop directly pops the top label of a
matching packet.
• If the egress advertises an explicit null label, the penultimate hop swaps the top label value of a
matching packet with the explicit null label.