The MPLS module assigns an SR-TE LSP the maximum LSP metric value of 16 777 215 when the local router provides the hop-to-label translation for its path. For an SR-TE LSP that uses PCE for path computation (pce-computation option enabled) by the PCE and/or has its control delegated to the PCE (pce-control enabled), the latter will return 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 an SR-TE LSP is derived from the MTU of the outgoing SR shortest path tunnel it is using, 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 a particular IGP instance using config>router>ospf/isis>segment-routing>tunnel-mtu. If no value was configured by the user, the SR tunnel MTU will be fully determined by the IGP interface calculation. This calculation is performed by the IGP and passed to the SR module each time it changes due to an updated resolution of the node SID.
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 1 if segment-routing and remote-lfa options are enabled in the IGP instance; otherwise, it is set to 0.
This calculation is performed by the IGP and passed to the SR module each time it changes due to an updated resolution of the node SID.
The 7705 SAR also provides the MTU for the adjacency SID tunnel because it is needed in an SR-TE LSP if the first hop in the ERO is an adjacency SID. In that case, the 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 that the SR-TE LSP is using. The 7705 SAR also provides the MTU for the adjacency SID tunnel because it is needed in an SR-TE LSP if the first hop in the ERO is an adjacency SID. In that case, the calculation for SR_Tunnel_MTU (given above), initially introduced for a node SID tunnel, is applied to get the MTU of the adjacency SID tunnel.
numLabels is the number of labels found in the super NHLFE of the SR-TE LSP. At LER, the super NHLFE is pointing to the SR tunnel NHLFE, which has a primary and a backup NHLFE.
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 using is updated.