6
format. As a best practice, use the
IP address of a loopback interface
as an LSR ID.
3. Enter the view of the interface
that needs to perform MPLS
forwarding.
interface
interface-type
interface-number
N/A
4. Enable MPLS for the interface.
mpls enable
By default, MPLS is disabled on an
interface.
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 does the following:
a. Removes the label stack from the packet.
b. Fragments the IP packet. The length of a fragment is the MPLS MTU minus the length of the
label stack.
c. 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:
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 L2VPN or 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 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.
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