The implementation of SPB on SR OS uses the PBB data plane. There is no flooding of B-MAC based traffic. If a B-MAC is not found in the FDB, traffic is dropped until the control plane populates that B-MAC. Unicast B-MAC addresses are populated in all FDBs regardless of I-SID membership. There is a unicast FDB per B-VPLS both control B-VPLS and user BVPLS. B-VPLS instances that do not have any I-VPLS, have only a default multicast tree and do not have any multicast MFIB entries.
The data plane supports an ingress check (reverse path forwarding check) for unicast and multicast frames on the respective trees. Ingress check is performed automatically. For unicast or multicast frames the B-MAC of the source must be in the FDB and the interface must be valid for that B-MAC or traffic is dropped. The PBB encapsulation (See PBB Technology) is unchanged from current SR OS. Multicast frames use the PBB Multicast Frame format and SPBM distributes I-VPLS I-SIDs which allows SPB to populate forwarding only to the relevant branches of the multicast tree. Therefore, SPB replaces both spanning tree control and MMRP functionality in one protocol.
By using a single tree for multicast the amount of MFIB space used for multicast is reduced (per source shortest path trees for multicast are not currently offered on SR OS). In addition, a single tree reduces the amount of computation required when there is topology change.