MPLS MTU Setting

When nodes are properly communicating, use the default MTU value.

To change an MPLS MTU on a specific interface, run the mpls mtu mtu-value command in the interface view. An MPLS MTU change on a specific interface does not affect the interface MTU, whereas an interface MTU change causes the effective MPLS MTU to update.

Suggestion for MPLS MTU Setting:

• Pay attention to the fragment mechanism for MPLS MTU on the NetEngine 8000 F. The MPLS MTU on the interface takes effect only on IP traffic that enters MPLS networks.
• A too small MPLS MTU value leads to a great number of IP fragments, which deteriorates device performance or even causes packet loss.
• If an MPLS MTU is greater than an interface MTU, the interface MTU value takes effect.
• The interface MTU values on core routers' NNIs must be greater than those on core routers' UNIs so that the NNIs can forward labeled packets sent by the UNIs. Number of labels carried in an MPLS packet in various scenarios are as follows.

  MPLS VPN Scenario	Number of Labels	Description
  Intra-AS VPN	One VPN label and N public network labels.	Value of N (Depending on the Public-Network Tunnel Type): N is 1 when packets are transmitted on an LDP LSP. N is 1 when packets are transmitted on a static LSP. N is 1 when packets are transmitted on a TE tunnel. N is 2 when packets are transmitted on a TE tunnel in the LDP over TE scenario. N is 3 when packets are transmitted on a TE fast reroute (FRR) bypass tunnel in the LDP over TE scenario. NOTE: The preceding Ns take effect when the PHP function is disabled. If PHP is enabled and performed, N minus 1 (N - 1) takes effect.
  Inter-AS VPN Option A	A packet transmitted within an autonomous system (AS) carries one VPN label and N public network labels. A packet transmitted between ASs caries no labels.
  Inter-AS VPN Option B	A packet transmitted within an AS carries one VPN label and N public network labels. A packet transmitted between ASs caries one VPN label.
  Inter-AS VPN Option C	A packet sent within an original AS carries one VPN label, one Border Gateway Protocol (BGP) label, and N public network labels. A packet transmitted from the original AS to another AS carries one VPN label and one BGP label. A packet is transmitted in non-original ASs, the packet carries one VPN label and N public network labels.
  HoVPN andHVPLS	A packet transmitted within the core layer carries an inner label and N public network labels. A packet transmitted between a user-end provider edge (UPE) and a superstratum provider edge (SPE) carries one inner label.

• The MPLS MTU value must be less than or equal to the interface MTU value. To enable core routers to support more types of labeled packets, increase interface MTU values, not MPLS values.