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3.4.1. Parallel Adjacencies

Adj-SIDs can be used in order to represent a set of parallel interfaces between two adjacent routers.

A node MUST install a FIB entry for any locally originated Adj-SID of value W attached to a set of links B with:

Incoming Active Segment: W
Ingress Operation: NEXT
Egress interfaces: load-balance between any data-link within set B

When parallel adjacencies are used and associated with the same Adj-SID, and, in order to optimize the load-balancing function, a "weight" factor can be associated with the Adj-SID advertised with each adjacency. The weight tells the ingress (or an SDN/orchestration system) about the load-balancing factor over the parallel adjacencies. As shown in Figure 3, A and B are connected through two parallel adjacencies

                          Link-1
                        +--------+
                        |        |
                    S---A        B---C
                        |        |
                        +--------+
                          Link-2

           Figure 3: Parallel Links and Adj-SIDs

Node A advertises following Adj-SIDs and weights:

  • Link-1: Adj-SID 1000, weight: 1

  • Link-2: Adj-SID 1000, weight: 2

Node S receives the advertisements of the parallel adjacencies and understands that by using Adj-SID 1000 node A will load-balance the traffic across the parallel links (Link-1 and Link-2) according to a 1:2 ratio i.e., twice as many packets will flow over Link-2 as compared to Link-1.

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