The following are examples of MTU settings of the SRv6 origination FPE interface (interface-b).
All default values
By default, LFA is disabled in IS-IS. The network interface default MTU is 9786 bytes based on default FP4 Ethernet MTU of 9800 bytes.
The SRv6 origination FPE interface MTU defaults to the same value of 9786 bytes; there is no need for further adjustments.
All default values and RLFA or TI-LFA enabled
The user must adjust the SRv6 origination FPE interface MTU from the first bullet item case to account for the 24 bytes introduced by LFA on the FP4 hardware.
SRv6 origination FPE Interface MTU is 9786 - 24 = 9762 bytes.
A remote constrained network interface and RLFA or TI-LFA enabled
Assume a specific remote network interface in the SRv6 network is limited to a maximum value of 1500 bytes. The user must adjust the SRv6 origination FPE interface MTU in all the ingress PEs from the first bullet item case, to account for that constrained value, plus the 24 bytes introduced by LFA repair tunnel (Users can adjust for as many LFA SRHs they wish. The following example assumes 1 LFA SRH).
SRv6 origination FPE Interface MTU = 1500 - 24 = 1476 bytes.