Configuring IS-IS Flex-Algorithm for SRv6

SRv6 introduces flexible algorithms to the IPv6 data plane.

A router is provisioned with topology or algorithm-specific locators for each topology or algorithm pair supported by that node. Each locator is a covering prefix for all SIDs provisioned on that router which have the matching topology or algorithm. Locators associated with flexible algorithms are not advertised in a Prefix Reachability TLV (236 or 237). However, locators associated with algorithm 0 are advertised in a Prefix Reachability TLV (236 or 237), which allows legacy routers that do not support SRv6 to install a forwarding entry for algorithm 0 SRv6 traffic.

Each SRv6 locator is associated with an algorithm (either algorithm 0 or a flexible algorithm in the range of 128 to 255) and each algorithm represents a topologically-constrained forwarding construct. The M-flag within the flexible algorithm prefix metric sub-TLV is not applicable to prefixes advertised as SRv6 locators. The metric field in the locator TLV is used regardless of the M-flag in the FAD advertisement.

A router configured to participate in a flexible algorithm must use the selected FAD to compute the corresponding routing table. The available options are described below:

Algorithm 0 (legacy routing table entries) is constructed from information advertised as a traditional IP Reachability TLV or as an SRv6 locator (tlv27). When IP reach TLV and SRv6 locator TLV contain conflicting information, then the IP reach TLV information is used.
Algorithms ranging from 128 to 255 (Flex-Algorithm routing table entries) are constructed from information advertised and constructed from locators found in the SRv6 locator TLV (tlv27).

For route leaking of flexible algorithm-aware SRv6 locators between IS-IS areas, the following rules apply when a topology TLV is leaked (IP Reachability TLV or SRv6 locator TLV) including leaked locators and end SIDs:

For algorithm 0, this SRv6 locator route is programmed as regular IS-IS route. If an IS-IS route is readvertised and has also an SRV6 locator TLV, it is readvertised as a regular IP Reachability TLV and SRv6 locator TLV.
For algorithms ranging from 128 to 255, if locator leaking is enabled, the original SRv6 locator TLV is readvertised as a SRv6 locator TLV into the other area.
Default locator leaking behavior between levels:
- For Level 1 to Level 2, leaking is enabled by default.
- For Level 2 to Level 1, leaking is disabled by default.
- Changing the default leaking behavior requires an export policy where the prefix-list keyword behavior is enhanced to match upon prefixes or locators found in the routing table regardless of the associated algorithm. The prefix-list allows combined support for algorithm 0 locators (regular prefixes) and algorithm locators ranging from 128 to 255 (Flex-Algorithm prefixes).
```
Configure
+---router
    +---policy-options
        +---[no] policy-statement <name>
            +---from
                +---[no] level [1|2]
                +---[no] prefix-list
                +---[no] protocol
                +---[no] tag
            +---to
	                +---<snip>
```

If a locator is associated with a flexible algorithm and the LFA is enabled, then LFA paths to the locator prefix must be calculated using the flexible algorithm in the corresponding topology to guarantee that they follow the same constraints as the calculation of the primary paths. LFA paths must only use SRv6 SIDs advertised specifically for the flexible algorithm. The LFA configuration is inherited from algorithm 0. The anycast behavior of SRv6 flexible algorithms is inherited from the standard algorithm 0 (standard SPF) SRv6 configuration.

The IS-IS neighbor advertisements are topology-specific and not algorithm-specific. Therefore, the SRv6 End.X SIDs inherit topology from the associated neighbor advertisement, but the algorithm is specified in the individual SID. All End.X SIDs are a subnet of a locator with matching topology and algorithm which is advertised by the same node in an SRv6 locator TLV. The End.X SIDs which do not meet this requirement are ignored. All End.X SIDs must find a supernet by the subnet of a locator with the matching algorithm which is advertised by the same router in an SRv6 locator TLV. The End.X SIDs which do not meet this requirement are ignored.

IS-IS protocol limitations affect enabling SRv6 flexible algorithms on a broadcast network. On a broadcast network, the LAN End.X SIDs of all neighbors for all participating flexible algorithms need to be advertised in a single LSP fragment because each IS-IS TE-NBR with all its TLV blocks must be advertised in one IS-IS LSP fragment. The amount of information inserted by segment routing for SRv6 into the LSP fragment depends upon the number of the flexible algorithms used, the number of static or auto-end.X configured per locator, and if both SRv6 and SR-MPLS are deployed.