IS-IS control plane changes

When a local locator is enabled in the MT2 IPv6 unicast topology of an IS-IS instance, IS-IS advertises the following routes:

• The prefix itself in a top level Multi-Topology Reachable IPv6 Prefixes TLV type 237 with the MT-ID field set to 2. This TLV is supported in SR OS.
• A top level locator TLV (top level SRv6 Locator TLV type 27) that contains the locator prefix as well as the End SID sub-TLVs associated with this local locator. The locator TLV is advertised with the MT-ID field set to 2.
• The End.X or LAN End.X sub-TLV in the top level multi-topology Extended IS Reachability TLV (222).
• The TE link application specific attributes sub-TLV in the top level multi-topology Extended IS Reachability TLV (222). This is to support the flex-algo feature.
• The Application-Specific Shared Risk Link Group (SRLG) TLV (238)
  Table: SRv6 IS-IS MT2 TLVs summarizes the new or modified TLVs in support of SRv6 in IS-IS MT2.

  Table: SRv6 IS-IS MT2 TLVs

  Multi-topology TLV	Codepoint	IS-IS context TLV	Description	SR support
  Multi-topology IPv6 Reach TLV	237	Is a top Level IS-IS TLV	This is the main prefix TLV MT-ID field set to 2	Yes
  SRv6 Locator TLV	27	Is a top Level IS-IS TLV	Indicates the locator, configured on this node, is used to terminate SIDs in algorithm 0 and flex-algo 128-255 MT-ID field set to 2	Yes
  SRv6 End SID Sub-TLV	5	SRv6 Locator TLV	Advertises the SID for the endpoint or End function (equivalent of prefix SID in SR-MPLS)	Yes
  Prefix Attribute Flags Sub-TLV	4	SRv6 Locator TLV (also in IPv6 Reach TLV 237)	Indicates the prefix of the locator is anycast	Yes
  SRv6 End.X SID Sub-TLV	43	Top level Multi-topology Extended IS Reachability TLV (222)	Advertises the SID for the adjacency or End.X function over a P2P link (equivalent of adjacency SID sub-TLV for P2P link in SR-MPLS)	Yes
  SRv6 LAN End.X SID sub-TLV	44	Top level Multi-topology Extended IS Reachability TLV (222)	Advertises the SID for the adjacency or End.X function over a LAN (equivalent of adjacency SID sub-TLV for LAN in SR-MPLS)	Yes
  SRv6 SID Structure Sub-Sub-TLV	1	SRv6 End SID Sub-TLV, SRv6 End.X SID Sub-TLV, SRv6 LAN End.X SID Sub-TLV	Provides the length of each field (Block, Locator, Function, and Argument) of the SRv6 SID it is advertised with	No SR OS does not advertise this sub-sub-TLV. If received from other vendor's implementation, it is not displayed in Link-State database and is also not propagated with the locator TLV.
  Application Specific Link Attributes (ASLA) sub-TLV	16	Top level Multi-topology Extended IS reachability TLV (222)	Advertises the link attributes for the flex-algo application	Yes
  Application-Specific Shared Risk Link Group (SRLG) TLV	238	Is a top Level IS-IS TLV	Advertises the link SRLG attribute for the flex-algo application	No SRLG constraint is not supported in IS-IS MT0 or MT2. IS-IS does not advertise this TLV and does not use it for any puspose if received from the network.

In prior releases of SR OS, only local locator routes and local prefix routes were redistributed with an export policy between topologies MT0 and MT2 of the same IS-IS instance or different IS-IS instances. These local locators were redistributed as a prefix TLVs and not as a locator TLVs. In addition, the tag of the local locator was not redistributed. Routes of remote locators and remote prefixes were not redistributed.

Redistribution between MT0 and MT2 topologies of the same IS-IS instance is not allowed for both remote locator and remote prefix routes.