SR-TE LSP metric and MTU settings

The MPLS module assigns a TE-LSP the maximum LSP metric value of 16777215 when the local router provides the hop-to-label translation for its path. For a TE-LSP that uses the local CSPF or the PCE for path computation (path-computation-method pce option enabled) by PCE and, or which has its control delegated to PCE (pce-control enabled), the latter returns the computed LSP IGP or TE metric in the PCReq and PCUpd messages. In both cases, the user can override the returned value by configuring an admin metric using the command config>router>mpls>lsp>metric.

• The MTU setting of a SR-TE LSP is derived from the MTU of the outgoing SR shortest path tunnel it is riding, adjusted with the size of the super NHLFE label stack size.

  The following are the details of this calculation:

  SR_Tunnel_MTU = MIN {Cfg_SR_MTU, IGP_Tunnel_MTU- (1+frr-overhead)*4}

  Where:

  • Cfg_SR_MTU is the MTU configured by the user for all SR tunnels within an IGP instance using config>router>ospf/isis>segment-routing>tunnel-mtu. If no value was configured by the user, the SR tunnel MTU is fully determined by the IGP interface calculation (described below).

  • IGP_Tunnel_MTU is the minimum of the IS-IS or OSPF interface MTU among all the ECMP paths or among the primary and LFA backup paths of this SR tunnel.

  • frr-overhead is set to:

    • value of ti-lfa [max-sr-frr-labels labels] if loopfree-alternates and ti-lfa are enabled in this IGP instance

    • 1 if loopfree-alternates and remote-lfa are enabled but ti-lfa is disabled in this IGP instance

    • 0 for all other cases

  This calculation is performed by IGP and passed to the SR module each time it changes because of an updated resolution of the node SID.

  SR OS also provides the MTU for adjacency SID tunnel because it is needed in a SR-TE LSP if the first hop in the ERO is an adjacency SID. In that case, this calculation for SR_Tunnel_MTU, initially introduced for a node SID tunnel, is applied to get the MTU of the adjacency SID tunnel.

• The MTU of the SR-TE LSP is derived as follows:

  SRTE_LSP_MTU= SR_Tunnel_MTU- numLabels*4

  Where:

  • SR_Tunnel_MTU is the MTU SR tunnel shortest path the SR-TE LSP is riding. The formula is as specified above.

  • numLabels is the number of labels found in the super NHLFE of the SR-TE LSP. Note that at LER, the super NHLFE is pointing to the SR tunnel NHLFE, which itself has a primary and a backup NHLFEs.

  This calculation is performed by the SR module and is updated each time the SR-TE LSP path changes or the SR tunnel it is riding is updated.

  Note: The above calculated SR-TE LSP MTU is used for the determination of an SDP MTU and for checking the Layer 2 service MTU. For the purpose of fragmentation of IP packets forwarded in GRT or in a VPRN over a SR-TE LSP, the data path always deducts the worst case MTU (12 labels) from the outgoing interface MTU for the decision to fragment or not the packet. In this case, the above formula is not used.