R2511-HP MSR Router Series MPLS Configuration Guide(V5)
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Configuring MPLS fast forwarding
MPLS fast forwarding is designed to improve MPLS forwarding efficiency. With the idea of "route once,
switch many," MPLS fast forwarding forwards the first packet of a data stream based on the normal MPLS
forwarding process and, during this period, creates a fast forwarding entry to record the label stack and
the link layer header of the packet. Then, MPLS fast forwarding forwards all subsequent packets of the
data stream based on the fast forwarding entry, encapsulating the packets with the same link layer
header as the first packet. This greatly improves MPLS forwarding efficiency.
MPLS fast forwarding depends on IP fast forwarding. IP fast forwarding must be enabled for MPLS fast
forwarding to work. For information about IP fast forwarding, see Layer 3—IP Services Configuration
Guide.
To configure MPLS fast forwarding:
Ste
p
Command
Remarks
1. Enter system view.
system-view N/A
2. Enter interface view.
interface interface-type
interface-number
N/A
3. Enable fast forwarding in the
inbound or outbound
direction.
ip fast-forwarding [ inbound |
outbound ]
Optional.
By default, fast forwarding is
enabled in both directions.
Configuring MPLS MTU
An MPLS label stack is inserted between the link layer header and the network layer header. During
MPLS forwarding, a packet, after encapsulated with an MPLS label, might exceed the allowed length of
the link layer and cannot be forwarded, although the network layer packet is smaller than the interface
MTU.
To address the issue, you can configure the MPLS MTU on an interface of an LSR. Then, the LSR compares
the length of an MPLS packet against the configured MPLS MTU on the interface. If 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:
Ste
p
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.
The following applies when an interface handles MPLS packets: