Tunnel MTU determination

The MTU of an SR tunnel populated into TTM is determined like an IGP tunnel; for example, LDP LSP is based on the outgoing interface MTU minus the label stack size. Segment routing, however, supports remote LFA and TI-LFA, which can program an LFA repair tunnel by adding one or more labels.

To configure the MTU of all SR tunnels within each IGP instance:

config>router>isis>segment-routing>tunnel-mtu bytes bytes
config>router>ospf>segment-routing>tunnel-mtu bytes bytes

There is no default value for this command. If the user does not configure an SR tunnel MTU, the MTU is determined by IGP as described below.

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 the CLI shown above. 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 the following parameters:
- value of ti-lifa [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
- otherwise, it is set to 0

The SR tunnel MTU is dynamically updated anytime any of the parameters used in its calculation change. This includes when the set of the tunnel next-hops changes or the user changes the configured SR MTU or interface MTU value.

Note: The calculated SR tunnel MTU is used to determine an SDP MTU and to check the Layer 2 service MTU. When fragmenting IP packets forwarded in GRT or in a VPRN over an SR shortest path tunnel, the data path always deducts the worst-case MTU (5 labels or 6 labels if hash label feature is enabled) from the outgoing interface MTU when deciding whether to fragment the packet. In this case, the above formula is not used.