An adjacency set is a bundle of adjacencies, represented by a common adjacency SID for the bundled set. It enables, for example, a path for an SR-TE LSP through a network to be specified while allowing the local node to spray packets across the set of links identified by a single adjacency SID.
SR OS supports both parallel adjacency sets (for example, those where adjacencies originating on one node terminate on a second, common node), and the ability to associate multiple interfaces on a specified node, irrespective of whether the far end of the respective links of those interfaces terminate on the same node.
An adjacency set is created under IS-IS or OSPF using the following CLI commands:
config
router
isis | ospf
segment-routing
[no] adjacency-set id
family [ipv4 | ipv6]
parallel [no-advertise]
no parallel
exit
...
. exit
exit
config
router
ospf
segment-routing
[no] adjacency-set id
parallel [no-advertise]
no parallel
exit
...
.
exit
The adjacency-set id command specifies an adjacency set, where id is an unsigned integer from 0 to 4294967295.
In IS-IS, each adjacency set is assigned an address family, IPv4 or IPv6. The family command for IS-IS indicates the address family of the adjacency set. For OSPF, the address family of the adjacency set is implied by the OSPF version and the instance.
The parallel command indicates that all members of the adjacency set must terminate on the same neighboring node. When the parallel command is configured, the system generates a trap message if a user attempts to add an adjacency terminating on a neighboring node that differs from the existing members of the adjacency set. See Associating an interface with an adjacency set for details about how to add interfaces to an adjacency set. The system stops advertising the adjacency set and deprograms it from TTM. The parallel command is enabled by default.
By default, parallel adjacency sets are advertised in the IGP. The no-advertise option prevents a parallel adjacency set from being advertised in the IGP; it is only advertised if the parallel command is configured. To prevent issues in the case of ECMP if a non-parallel adjacency set is used, an external controller may be needed to coordinate the label sets for SIDs at all downstream nodes. As a result, non-parallel adjacency sets are not advertised in the IGP. The label stack below the adjacency set label must be valid at any downstream node that exposes it, even though it is sprayed over multiple downstream next-hops.
Parallel adjacency sets are programmed in TTM (unless there is an erroneous configuration of a non-parallel adjacency). Non-parallel adjacency sets are not added to TTM or RTM, meaning they cannot be used as a hop at the originating node. Parallel adjacency sets that are advertised are included in the link-state database and TE database, but non-parallel adjacency sets are not included because they are not advertised.
An adjacency set with only one next hop is also advertised as an individual adjacency SID with the S flag set. However, the system does not calculate a backup for an adjacency set even if it has only one next hop.