IS-IS control plane extensions

The following CLI enables SRv6 in the IS-IS instance and assigns a locator to each algorithm (zero or flexible algorithm).

configure
+--router
+---isis <0..31>
|   +---segment-routing-v6
|   |   +---locator <locator-name>
|   |   |   +--level-capability <level-1|level-2|level-1/2>
|   |   |   +---level <1|2>
|   |   |   |   +---metric <1..16777215>
|   |   |   +---tag <1..4294967295>
|   |   +---shutdown
|   |   |   no shutdown

The IS-IS control plane extensions in support of SRv6 are defined in draft-ietf-lsr-isis-srv6-extensions.

The IS-IS control plane advertises the SRv6 capabilities sub-TLV and the SRv6 Locator TLV. This includes the End function sub-TLV (equivalent to the prefix SID sub-TLV in SR-MPLS) and the End.X function sub-TLV (equivalent to the adjacency SID sub-TLV for a P2P link and a LAN in SR-MPLS).

The weight field in the End.X or LAN End.X sub-TLVs is not filled in on transmit and is ignored on receipt of the link TLV.

The following table describes the supported IS-IS SRv6 TLVs in SR OS.

When both SR-MPLS and SRv6 are enabled on the same IS-IS instance, an MPLS node SID cannot be configured for a prefix of the locator or an End SID. This is because the SRv6 locator subnet cannot be added to a network interface and an MPLS node SID is configurable against a network interface only.

However, both the SRv6 locator route and the SR-MPLS tunnel are programmed if IS-IS receives from a third-party router implementation, a /128 prefix that has both a locator TLV and a prefix SID TLV (with node flag enabled). If the prefix SID is for a subnet larger than /128, only the locator route is programmed and the SR-MPLS tunnel is not.

Each SID function encoded in a SRv6 SID has its own endpoint behavior codepoint as listed in Table: SRv6 SID function endpoint behavior codepoints.

SR OS advertises service SID functions in the BGP control plane only as described in BGP Service control plane extensions.

For the SRH processing and removal at the SID termination , the following modes of operation are associated with the termination of the End or End.X SID. These modes are sometimes referred to as SID flavours and IS-IS assigns a unique codepoint for each mode of the same End or End.X SID.

Basic or unflavored mode

The router that terminates an End or End.X SID and the Segments-Left field in the received packet is 0 before decrementing, keeps the SRH in the packet, and processes the packet identified by the next-header in the SRH. Typically, the next-header indicates another SRH and the packet is then forwarded based on the lookup of that next-SID; SR OS does not support this mode.

Ultimate SRH Popping (USP) mode

The egress PE terminates the last End or End.X segment in the outer IPv6 header and then processes the service SID in the SRH. SR OS supports this mode.

Penultimate SRH Popping (PSP) mode

The router that terminates the End or End.X segment before the last in the segment list, meaning the Segments-Left field before decrementing has a value of 1, removes the SRH on behalf of the egress PE. SR OS supports this mode.

PSP&USP mode

This is a combination of both the USP and PSP modes. The router that terminates the End or End.X segment applies the corresponding behavior for value 0 and 1 of the Segments-Left field. SR OS does not support this mode.

Ultimate Segment Decapsulation (USD) mode

This is a variant of the USP in which the node skips the SRH and moves directly to process the next header indicated in the SRH. SR OS does not support this mode.