The MTU of an SR tunnel populated into the TTM is determined the same way as it is for an IGP tunnel; for example, LDP LSP, based on the outgoing interface MTU minus the label stack size. Segment routing, however, supports remote LFA, which programs an LFA backup next-hop adding another label to the tunnel for a total of two labels.
The user must configure the MTU of all SR tunnels within each IGP instance:
configure>router>isis>segment-routing>tunnel-mtu bytes
configure>router>ospf>segment-routing>tunnel-mtu bytesThere is no default value for this new command. If the user does not configure an SR tunnel MTU, the MTU is fully determined by IGP.
The MTU of the SR tunnel, in bytes, is determined as follows:
Where:
Cfg_SR_MTU is the MTU configured by the user for all SR tunnels within a specific IGP instance using the preceding commands. If no value was configured by the user, the SR tunnel MTU is determined by the IGP interface calculation.
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.
The SR tunnel MTU is dynamically updated whenever any of the preceding parameters changes. This includes when the set of the tunnel next-hops changes, or the user changes the configured SR MTU or interface MTU value.
For the purpose of fragmentation of IP packets forwarded in GRT or in a VPRN over an SR shortest path tunnel, the IOM always deducts the worst case MTU (5 labels or 6 labels if the hash label feature is enabled) from the outgoing interface MTU for the decision to fragment the packet or not. In this case, the preceding formula is not used.