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Handling of the IS-IS Binding SID S Flag and RFC 7794 Prefix Attribute Flags

As part of the SR support, the SID/Label Binding TLV originates on any router in an IS-IS domain. There are multiple uses of the SID/Label Binding TLV. One use case is that the SID/Label Binding TLV could be used to advertise prefixes to SID/Label mappings. This functionality is called the Segment Routing Mapping Server (SRMS). LDP mapping client/server is the feature that enables LDP and SR, interoperate in a network with islands of LDP and SPRING capable routers.

Any router in the domain could be configured as SRMS server. To leak binding SIDs across levels you need to configure one node as SRMS server in each IS-IS level. If the S flag is set, then the SRMS binding SIDs are leaked across the levels by the ABR (Area Border Router).

IS-IS binding SIDs supports the following in IS-IS SRMS:

  • To leak binding SIDs across IS-IS levels if the S flag is set.
  • Setting and resetting of A, D flags when binding SIDs are leaked across levels.
  • Support prefix attribute flags sub TLV of RFC 7794.

Benefits of IS-IS binding SID S flag and RFC 7794 prefix attribute flags:

  • Advertise binding SIDs in single node in the entire domain, rather than configuring and advertising at each level.
  • Allows you to disable the binding SIDs leakage across IS-IS levels.
  • Ability to control the binding SIDs leaking and determine the traffic route with labels/prefixes by setting the A and D flags.
  • Set the prefix attribute flags on the receiving router to have accurate information on the routes.

The IS-IS leaks the binding SIDs by default if the S flag is set. You can disable IS-IS binding SIDs leak by including the no-binding-sid-leaking configuration statement at the [edit protocols isis source-packet-routing] hierarchy level. To configure and support the encoding of the prefix attribute flags sub tlv include the prefix-attribute-flags configuration statement at the [edit policy-options policy-statement <policy-name> then] hierarchy level. You can use the options [node-segment-flag | external-prefix-flag] for the prefix-attribute-flags configuration statement.

Figure 1: IS-IS binding SID 'S' flag and RFC 7794 prefix attribute flags IS-IS binding SID 'S' flag and RFC 7794 prefix attribute flags

In this topology, R0, R1, and R2 are configured with IS-IS Segment Routing protocol. R3 is configured with IS-IS, SR and LDP. R4 is configured with IS-IS and LDP, but without SR. R1 and R2 are Area Border Routers (ABRs).

R0 participates at Level-1 of IS-IS domain. R1 and R2 participates with IS-IS SR at Level 1 and Level 2. R3 participates in Level 1 and advertises the binding SIDs by acting as a SRMS server. R2 leaks the binding SIDs to level 2. R1 leaks the binding SIDs to level 1.

You can set the S flag to allow the label binding TLV to leak through the IS-IS levels (Level 1 or Level 2). You can set the A flag to program the penultimate-hop popping. During leaking binding SID, the D flag is set to prevent the leaking of the label binding TLV from Level 2 back to Level 1.