Programming ILM/NHLFE for the same prefix across IGP instances

Figure: Handling of same prefix and SID in different IS-IS instances shows an IS-IS example of handling in case of a global SID index range.

In global SID index range mode of operation, the resulting ILM label value is the same across the IGP instances. The router programs ILM/NHLFE for the prefix of the winning IGP instance based on the RTM route-type preference. The router generates both a trap message and a syslog error message, and does not program the other prefix SIDs in the datapath.

In per-instance SID index range mode of operation, the resulting ILM label has different values across the IGP instances. The router programs ILM/NHLFE for each prefix as expected.

Figure: Handling of same prefix and SID in different IS-IS instances

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 E 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 is one SR tunnel entry for L2 in TTM:
  - tunnel-id 1: prefix N-SIDx
  - nhop1 = B
  - nhop2 = C
  - tunl-pref 10