In SPBM, BMAC addresses are carried within the IS-IS link-state database. To do this, SPBM supports an IS-IS TLV that advertises the I-SID and BMAC information across the network. Each node has a System ID, which also serves as Backbone MAC address (BMAC) of the switch. These Backbone MAC addresses are populated into the SPBM VLAN Forwarding Information Base (FIB).
SPBM runs all pair Dijkstras to produce the multicast FIB. The computing node loops through each node to run Dijkstra using that node as the root, and then prunes paths to only keep the shortest paths. The computing node then computes the intersection of the set of I-SIDs for which the root node transmits, with the set of I-SIDs for which the path endpoints receive.
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.
The multicast addresses are built out of two pieces: the SPBM Node Nickname and the I-SID ID converted to hexadecimal format to form the multicast MAC address.
|----------------------------|---------------------------------|
nickname|0x30000 hexadecimal I-SID
For example, if the nickname is 0.00.10 and the I-SID is 100 (0x64), the multicast address is 03:00:10:00:00:64.
The multicast FIB is not produced until virtual services are configured and learned.