When segment routing is enabled in the IS-IS or OSPF instance, the router performs the following operations. See Control protocol changes for more information about the TLVs and sub-TLVs for both IS-IS and OSPF protocols:
Advertises the segment routing capability sub-TLV to routers in all areas or levels of this IGP instance. However, only neighbors with which it established an adjacency interprets the SID or label range information and use it for calculating the label to swap to or push for a resolved prefix SID.
Advertises the assigned index for each configured node SID in the new prefix SID sub-TLV with the N-flag (node-SID flag) set. The segment routing module programs the incoming label map (ILM) with a pop operation for each local node SID in the datapath.
Automatically assigns and advertises an adjacency SID label for each formed adjacency over a network IP interface in the new adjacency SID sub-TLV. The following points should be considered:
The adjacency SID is advertised for both numbered and unnumbered network IP interfaces.
The adjacency SID for parallel adjacencies between two IGP neighbors is not supported.
The adjacency SID is not advertised for an IES interface because access interfaces do not support MPLS.
The adjacency SID must be unique for each instance and for each adjacency. Also, ISIS MT=0 can establish an adjacency for the IPv4 address family over the same link, and in such a case a different adjacency SID is assigned to each next-hop. However, the existing IS-IS implementation assigns a single protect-group ID (PG-ID) to the adjacency, and when the state machine of a BFD session tracking the IPv4 next-hop times out, an action is triggered for the prefixes of the IPv4 address family over that adjacency.
The segment routing module programs the ILM with a swap to an implicit null label operation for each advertised adjacency SID.
Resolve received prefixes and, if a prefix SID sub-TLV exists, the segment routing module programs the ILM with a swap operation and an LTN with a push operation, both pointing to the primary/LFA NHLFE. An SR tunnel is also added to the TTM. If a node SID resolves over an IES interface, the datapath is not programmed and a trap is raised. Therefore, only next-hops of an ECMP set corresponding to network IP interfaces are programmed in the datapath; next-hops corresponding to IES interfaces are not programmed. If, however, the user configures the interface as a network on one side and IES on the other side, MPLS packets for the SR tunnel received on the access side are dropped.
LSA filtering causes SIDs not to be sent in one direction, which means that some node SIDs are resolved in parts of the network upstream of the advertisement suppression.
When the user enables segment routing in an IGP instance, the main SPF and LFA SPF are computed normally and the primary next-hop and LFA backup next-hop for a received prefix are added to RTM without the label information advertised in the prefix SID sub-TLV. In all cases, the segment routing tunnel is not added into the RTM.