7
To set 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.
Set an MPLS MTU for the
interface.
mpls mtu
size
By default, no MPLS MTU is set
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.
18B
Specifying the label type advertised by egress
In an MPLS network, an egress node 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 node
determines how the penultimate hop processes a labeled packet.
•
If the egress node advertises an implicit null label, the penultimate hop directly pops the top
label of a matching packet.
•
If the egress node advertises an explicit null label, the penultimate hop swaps the top label
value of a matching packet with the explicit null label.
•
If the egress node advertises a non-null label, the penultimate hop swaps the top label of a
matching packet with the label assigned by the egress node.
323B
Configuration guidelines
As a best practice, configure the egress node to advertise an implicit null label to the penultimate hop
if the penultimate hop supports PHP. If you want to simplify packet forwarding on egress but keep
labels to determine QoS policies, configure the egress node to advertise an explicit null label to the
penultimate hop. Use non-null labels only in particular scenarios. For example, when OAM is
configured on the egress node, the egress node can get the OAM function entity status only through
non-null labels.
As a penultimate hop, the device accepts the implicit null label, explicit null label, or normal label
advertised by the egress node.
For LDP LSPs, the
mpls label advertise
command triggers LDP to delete the LSPs established
before the command is executed and re-establishes new LSPs.
For BGP LSPs, the
mpls label advertise
command takes effect only on the BGP LSPs established
after the command is executed. To apply the new setting to BGP LSPs established before the
command is executed, delete the routes corresponding to the BGP LSPs, and then redistribute the
routes.
