H3C Technologies H3C S6800 Series Switches - Manual (Page 20)

7
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.
• 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.
• If the egress advertises a non-null label (normal label), the penultimate hop swaps the top label of
a matching packet with the specific label assigned by the egress.
Configuration guidelines
If the penultimate hop supports PHP, H3C recommends that you configure the egress to advertise an
implicit null label to the penultimate hop. If you want to simplify packet forwarding on the egress but keep
labels to determine QoS policies, configure the egress to advertise an explicit null label to the
penultimate hop. H3C recommends using non-null labels only in particular scenarios. For example, when
OAM is configured on the egress, the egress 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 device.
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.