In SPBM, Backbone MAC (BMAC) addresses are carried within the IS-IS link-state database. To do this, SPBM supports an IS-IS Type-Length-Value (TLV) that advertises the Service Instance Identifier (I-SID) and BMAC information across the network. Each node has a System ID, which also serves as BMAC of the switch. These BMAC addresses are populated into the SPBM Forwarding Information Base (FIB).
When the network topology is discovered and stored in the IS-IS link-state database, each node calculates shortest path trees for each source node, so that a unicast path now exists from every node to every other node. With this information, each node populates unicast information received from SPBM into the FIB for forwarding purposes.
I-SIDs are only used for virtual services (Layer 2 VSNs and Layer 3 VSNs). If you only enable IP Shortcuts on the Backbone Edge Bridges, I-SIDs are never exchanged in the network as IP Shortcuts allow Global Routing Table (GRT) IP networks to be transported across IS-IS.
The show isis spbm ip-unicast-fib or show isis spbm ipv6-unicast-fib command displays all of the IS-IS routes in the IS-IS LSDB. The IP ROUTE PREFERENCE column in the show output displays the IP route preference.
Routes within the same VSN are added to the LSDB with a default preference of 7. Inter-VSN routes are added to the LSDB with a route preference of 200. IS-IS accept policies enable you to change the route preference for incoming routes. If the same route is learned from multiple sources with different route preferences, then the routes are not considered equal cost multipath (ECMP) routes. The route with the lowest route preference is the preferred route. In Layer 2, in the event of a tie-break between routes from multiple sources, the tie-breaking is based on cost and hop count.
The unicast computation runs a single Dijkstra (unlike all pair Dijkstras for multicast). SPBM produces only one Shortest Path First (SPF) tree and the tree is rooted on the computing node.
The unicast computation generates an entry for each node in the network. The Destination Address (DA) for that entry is the system-id of the node. In addition, if a node advertises MAC addresses other than the system-id, each MAC address has an entry in the unicast FIB table, and the shortest path to that MAC should be exactly the same as the path to the node.
Unicast FIB entries are installed to the vlan-fdb table.