H3C Technologies H3C SR8800 - Manual (Page 32)

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Use the following command to reset LDP sessions:
Task Command Remarks
Reset LDP sessions.
reset mpls ldp [ all | [ vpn-instance
vpn-instance-name ] [ fec mask |
peer peer-id ] ]
Available in user view
Managing and optimizing MPLS forwarding
Configuring MPLS MTU
An MPLS label stack is inserted between the link layer header and network layer header of a packet.
During MPLS forwarding, a packet, after encapsulated with an MPLS label, may exceed the allowed
length of the link layer and thus cannot be forwarded, although the network layer packet is smaller than
the MTU of the interface.
To address the issue, you can configure the MPLS MTU on an interface of an LSR. Then, the LSR will
compare the length of an MPLS packet against the configured MPLS MTU on the interface. When the
packet is larger than the MPLS MTU:
• If fragmentation is allowed, the LSR 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
back into each fragment, and then forwards the fragments.
• If fragmentation is not allowed, the LSR drops the packet directly.
To configure the MPLS MTU of 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 the MPLS MTU of
the interface.
mpls mtu value
By default, the MPLS MTU of an
interface is not configured.
NOTE:
•
MPLS packets carrying L2VPN or IPv6 packets are always successfully forwarded, even if they are larger
than the MPLS MTU.
• If the MPLS MTU of an interface is greater than the MTU of the interface, data forwarding may fail on the
interface.
• If you do not configure the MPLS MTU of an interface, fragmentation of MPLS packets will be based on
the MTU of the interface, and the length of fragments does not take the MPLS labels into account. Thus,
the length of an MPLS fragment may be larger than the interface’s MTU.
Configuring TTL processing mode at ingress
At the ingress of an LSP, a label stack encapsulated with a TTL field is added to each packet. Whether the
label TTL takes the IP TTL or not depends on whether IP TTL propagation is enabled.