Shared Risk Loss Group (SRLG) is used to tag IP interfaces which share a specific fate with the same identifier. For example, an SRLG group identifier could represent all links that use separate fibers but are carried in the same fiber conduit. If the conduit is accidentally cut, all the fiber links are cut which means all IP interfaces using these fiber links fail. Therefore, the user can enable the SRLG constraint to select a LFA next-hop for a prefix, which avoids all interfaces that share fate with the primary next.
The user first configures locally on each router the name and identifier of each SRLG group:
configure>router>if-attribute>srlg-group group-name value group-value
A maximum of 1024 SRLGs can be configured per system.
Next the user configures the admin group membership of the IP interfaces used in LFA. The user can apply SRLG groups to a network IP interface.
config>router>interface>if-attribute>srlg-group group-name[group-name...(up to 5 max)]
The user can add a maximum of 64 SRLG groups to a specific IP interface. The same preceding command can be applied multiple times.
Note that the configured SRLG membership is applied in all levels/areas the interface is participating in. The same interface cannot have different memberships in different levels/areas.
The no form of the srlg-group command under the interface deletes one or more of the SRLG memberships of the interface. It deletes all SRLG memberships if no group name is specified.
Finally, the user adds the SRLG constraint into the route next-hop policy template:
configure router route-next-hop-template template template-name
srlg-enable
When this command is applied to a prefix, the LFA SPF selects an LFA next-hop, among the computed ones, which uses an outgoing interface that does not participate in any of the SLRGs of the outgoing interface used by the primary next-hop.
Note the SRLG and admin-group criteria are applied before running the LFA next-hop selection algorithm. The modified LFA next-hop selection algorithm is shown in Section 7.5.