Supporting multiple topologies for the same destination prefix

The SR OS supports assigning different prefix-SID indexes and labels to the same prefix in different IGP instances. While other routers that receive these prefix SIDs program a single route into RTM, based on the winning instance ID as per RTM route type preference, the SR OS adds two tunnels to this destination prefix in TTM. This supports multiple topologies for the same destination prefix.

For example: in two instances (L2, IS-IS instance 1 and L1, IS-IS instance 2—see Figure: Programming multiple tunnels to the same destination), Router D has the same prefix destination (N) with different SIDs (SIDx and SIDy).

Figure: Programming multiple tunnels to the same destination

Assume the following route-type preference in RTM and tunnel-type preference in TTM are configured:

ROUTE_PREF_ISIS_L1_INTER (RTM) 15
ROUTE_PREF_ISIS_L2_INTER (RTM) 18
ROUTE_PREF_ISIS_TTM 10

Note: The TTM tunnel-type preference is not used by the SR module. It is put in the TTM and is used by other applications such as VPRN auto-bind and BGP shortcut to select a TTM tunnel.

Router A performs the following resolution within the single IS-IS instance 1, level 2. All metrics are the same, and ECMP = 2.
- For prefix N, the RTM entry is:
  - prefix N
  - nhop1 = B
  - nhop2 = C
  - preference 18
- For prefix N, the SR tunnel TTM entry is:
  - tunnel-id 1: prefix N-SIDx
  - nhop1 = B
  - nhop2 = C
  - tunl-pref 10
Add IS-IS instance 2 (Level 1) in the same configuration, but in routers A, B, and C only.
- For prefix N, the RTM entry is:
  - prefix N
  - nhop1 = B
  - preference 15
  RTM prefers L1 route over L2 route
- For prefix N, there are two SR tunnel entries in TTM:
  
  SR entry for L2:
  - tunnel-id 1: prefix N-SIDx
  - nhop1 = B
  - nhop2= C
  - tunl-pref 10
  SR entry for L1 is tunnel-id 2: prefix N-SIDy.