H3C Technologies H3C S12500-X Series Switches - Manual (Page 17)

6
Step Command Remarks
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 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:
Step 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 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 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. The value range for a non-null label is 16 to 1048575.
For LSPs established by a label distribution protocol, the label advertised by the egress determines how
the penultimate hop processes a labeled packet.