Programming ILM/NHLFE for the same prefix across IGP instances

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 logs a trap and generates a syslog error message, and does not program the other prefix SIDs in the datapath.

In the 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.

The following figure shows an IS-IS example of handling in case of a global SID index range.

Figure: Handling of the 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 level 2, IS-IS instance 1. 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 Level 1, IS-IS instance 2 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
  The 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