An Ethernet network usually learns MAC addresses as frames are sent through the switch. This process is called reverse learning and is accomplished through broadcast.
SPBM does not permit broadcast flooding of traffic on the B-VLAN to prevent looping accomplished through flooding packets with unknown destinations (although multicast traffic is supported). As such, MAC addresses must be distributed within SPBM. This is accomplished by carrying the necessary BMAC addresses inside the IS-IS link state database. To that end, SPBM supports an IS-IS TLV that advertises the I-SID and BMAC information across the network. This functionality enables the powerful end-point-provisioning of SPBM.
These Backbone MAC addresses are populated into the SPBM VLAN Forwarding Information Base (FIB) to maximize efficiency and to enable Reverse Path Forwarding Check (RPFC) to operate properly.