Configuring Mpls Mtu; Configuring A Ttl Processing Mode For An Lsr - HP HSR6800 Configuration Manual

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:
Step
1. Enter system view.
2. Enter interface view.
3. Configure the MPLS MTU of
the interface.
The following applies when an interface handles MPLS packets:
•
An interface always forwards MPLS packets carrying L2VPN packets, even if the MPLS packet
size exceeds the interface MPLS MTU. However, whether the forwarding succeeds depends on
the actual forwarding capacity of the interface.
•
An interface drops MPLS packets carrying IPv6 packets if the MPLS packet size exceeds the
interface MPLS MTU. At the same time, the device sends the interface MPLS MTU to the
sender in a "Packet Too Big" ICMPv6 message.
•
If the interface MPLS MTU is greater than the interface MTU, data forwarding might fail on the
interface.
•
If no MPLS MTU is configured, MPLS packet fragmentation is based on the interface MTU.
Because the length of fragments does not take the MPLS labels into account, an MPLS
fragment might exceed the interface's MTU.

Configuring a TTL processing mode for an LSR

An LSR can process the TTL field in the either of the following ways:
•
Enable TTL propagation—When an LSR labels a packet, it copies the TTL value of the
original packet (the IP TTL value of an IP packet or the TTL value in the top label of a labeled
packet) to the TTL field of the newly added label. When an LSR pops the stack-top label, it
copies the TTL value of the stack-top label to the TTL field of the original packet. In this mode,
the TTL value of a packet is decreased hop by hop when forwarded along the LSP. The tracert
result reflects the real path along which the packet has traveled.
Command
system-view
interface interface-type
interface-number
mpls mtu value
19
Remarks
N/A
N/A
By default, the MPLS MTU of an
interface is not configured.