IGP sets the metric of the SRv6 remote locator prefix route or that of a local adjacency SID route to the metric of the computed path of the corresponding route.
The metric of a local End SID route is set to 0; similar to any local route.
The metric of a BGP IPv4/IPv6 or VPN-IPv4/VPN-IPv6 route resolved to a SRv6 tunnel inherits the value of the locator prefix route metric.
The user should configure the network interfaces at the ingress PE and at transit P routers with an MTU value that accounts for the fixed IPv6 header (40 bytes) and the additional LFA SRHs (24 bytes each).
The SRv6 origination FPE interface (interface-b) at ingress PE requires special attention. The datapath accounts for the fixed 40-byte IPv6 header when checking for fragmenting IPv4 packets (DF=0) or dropping IPv4 (DF=1) and IPv6 packets that are intended to be forwarded over an SRv6 tunnel. If LFA is enabled in IS-IS, the LFA overhead is not accounted for, and therefore the configured MTU for the SRv6 origination FPE interface must account for it.
In addition, there are network deployments where it is not possible to modify the network interface MTU or to set all network interfaces to the same value. In that case, the user must configure the SRv6 origination FPE interface MTU to reflect this worst case MTU in the network, accounting fixed and variable LFA overhead. The following is the CLI to use for this purpose.
configure
+--fwd-path-ext
+--fpe <fpe-id> [create]
+--srv6 <origination | termination>
+--interface-b
+--mtu <1280-9786> The show router base icmp6 command has a global count for received “Packets Too Big” that captures the dropped packets on network interfaces, including SRv6 origination FPE interface, caused by MTU violation.