Profile 28 w/ Constraints and FRR

Continuing on from the previous lab, configure the default MDTs to exclude RED links and use FRR. Color the XR1-XR3 link red.

Answer

#XR1, XR3
segment-routing
 traffic-eng
  interface GigabitEthernet0/0/0/0.3013
   affinity
    name RED
   !
  !
  affinity-map
   name RED bit-position 1

#XR5-8
segment-routing
 traffic-eng
  on-demand color 10
   dynamic
    metric
     type igp
    !
    affinity
     exclude-any
      name RED
  !
  affinity-map
   name RED bit-position 1
!
multicast-routing vrf CUSTOMER add ipv4
 mdt default segment-routing mpls color 10 fast-reroute lfa

Explanation

In profile 27 (static p2mp policies), the policy was configured directly on the PCE, so the constraints and FRR was configured there as well.

In profile 28 and 29, FRR and constraints are configured on the PEs. These affect only the p2mp policies for which they act as a root. (The PE only asks the PCE for path computation for p2mp policies for which it is the root).

To configure constraints, you must create a normal on-demand SR-TE policy, and then associate that color with the default MDT. Additionally, FRR is enabled on the default MDT.

This lab asks us to avoid link color RED. On the PEs, we must add an ODN policy as follows. Any color can be used. Also note that we could have choosen another metric type besides igp.

segment-routing
 traffic-eng
  on-demand color 10
   dynamic
    metric
     type igp
    !
    affinity
     exclude-any
      name RED
  !
  affinity-map
   name RED bit-position 1

This color is associated with the default MDT for SR. Additionally, FRR is enabled for the default MDT.

multicast-routing vrf CUSTOMER add ipv4
 mdt default segment-routing mpls color 10 fast-reroute lfa

If we look at the policy on the PCE, we can see that FRR is enabled, and the affinity constraint was signaled. Note that all the nexthops have an exclamation point, indicating FRR is in use.

This is also reflected on the PE. Note the outgoing interface is “none” as TI-LFA is used to find the outgoing interface now.