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3.4. IGP-Adjacency Segment (Adj-SID)

The adjacency is formed by the local node (i.e., the node advertising the adjacency in the IGP) and the remote node (i.e., the other end of the adjacency). The local node MUST be an IGP node. The remote node may be an adjacent IGP neighbor or a non-adjacent neighbor (e.g., a forwarding adjacency, [RFC4206]).

A packet injected anywhere within the SR domain with a segment list {SN, SNL} where SN is the Node-SID of node N and SNL is an Adj-SID attached by node N to its adjacency over link L will be forwarded along the shortest path to N and then be switched by N, without any IP shortest-path consideration, towards link L. If the Adj-SID identifies a set of adjacencies, then the node N load-balances the traffic among the various members of the set.

Similarly, when using a global Adj-SID, a packet injected anywhere within the SR domain with a segment list {SNL}, where SNL is a global Adj-SID attached by node N to its adjacency over link L, will be forwarded along the shortest path to N and then be switched by N, without any IP shortest-path consideration, towards link L. If the Adj-SID identifies a set of adjacencies, then the node N does load-balance the traffic among the various members of the set. The use of global Adj-SID allows to reduce the size of the segment list when expressing a path at the cost of additional state (i.e., the global Adj-SID will be inserted by all routers within the area in their forwarding table).

An "IGP-Adjacency segment" or "Adj-SID" enforces the switching of the packet from a node towards a defined interface or set of interfaces. This is key to theoretically prove that any path can be expressed as a list of segments.

The encodings of the Adj-SID include a set of flags supporting the following functionalities:

  • Eligible for Protection (e.g., using IPFRR or MPLS-FRR). Protection allows that in the event the interface(s) associated with the Adj-SID are down, that the packet can still be forwarded via an alternate path. The use of protection is clearly a policy-based decision; that is, for a given policy protection may or may not be desirable.

  • Indication whether the Adj-SID has local or global scope. Default scope SHOULD be local.

  • Indication whether the Adj-SID is persistent across control plane restarts. Persistence is a key attribute in ensuring that an SR Policy does not temporarily result in misforwarding due to reassignment of an Adj-SID.

A weight (as described below) is also associated with the Adj-SID advertisement.

A node SHOULD allocate one Adj-SID for each of its adjacencies.

A node MAY allocate multiple Adj-SIDs for the same adjacency. An example is to support an Adj-SID that is eligible for protection and an Adj-SID that is NOT eligible for protection.

A node MAY associate the same Adj-SID to multiple adjacencies.

In order to be able to advertise in the IGP all the Adj-SIDs representing the IGP adjacencies between two nodes, parallel adjacency suppression MUST NOT be performed by the IGP.

When a node binds an Adj-SID V to a local data-link L, the node MUST install the following FIB entry:

Incoming Active Segment: V
Ingress Operation: NEXT
Egress Interface: L

The Adj-SID implies, from the router advertising it, the forwarding of the packet through the adjacency or adjacencies identified by the Adj-SID, regardless of its IGP/SPF cost. In other words, the use of adjacency segments overrides the routing decision made by the SPF algorithm.

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