IEEE 802.1aq standard SPB uses a source specific tree model. The standard model is more computationally intensive for multicast traffic because in addition to the SPF algorithm for unicast and multicast from a single node, an all pairs shorted path needs to be computed for other nodes in the network. In addition, the computation must be repeated for each ECT algorithm. While the standard yields efficient shortest paths, this computation is overhead for systems where multicast traffic volume is low. Ethernet VLL and VPLS unicast services are popular in PBB networks and the SR OS SPB design is optimized for unicast delivery using shortest paths. Ethernet supporting unicast and multicast services are commonly deployed in Ethernet transport networks. SR OS SPB Single tree multicast (also called shared tree or *,G) operates similarly today. The difference is that SPB multicast never floods unknown traffic.
The SR OS implementation of SPB with shortest path unicast and single tree multicast, requires only two SPF computations per topology change reducing the computation requirements. One computation is for unicast forwarding and the other computation is for multicast forwarding.
A single tree multicast requires selecting a root node much like RSTP. Bridge priority controls the choice of root node and alternate root nodes. The numerically lowest Bridge Priority is the criteria for choosing a root node. If multiple nodes have the same Bridge Priority, then the lowest Bridge Identifier (System Identifier) is the root.
In SPB the source-bmac can override the chassis-mac allowing independent control of tie breaking, The shortest path unicast forwarding does not require any special configuration other than selecting the ECT algorithm by configuring a B-VPLS use a FID with low-path-id algorithm or high-path-id algorithm to be the tiebreaker between equal cost paths. Bridge priority allows some adjustment of paths. Configuring link metrics adjusts the number of equal paths.
To illustrate the behavior of the path algorithms an example network is shown in Figure 1.
Assume that Node A is the lowest Bridge Identifier and the Multicast root node and all links have equal metrics. Also, assume that Bridge Identifiers are ordered such that Node A has a numerically lower Bridge identifier than Node B, and Node B has lower Bridge Identifier than Node C, and so on, Unicast paths are configured to use shortest path tree (SPT). Figure 2 shows the shortest paths computed from Node A and Node E to Node F. There are only two shortest paths from A to F. A choice of low-path-id algorithm uses Node B as transit node and a path using high-path-id algorithm uses Node D as transit node. The reverse paths from Node F to A are the same (all unicast paths are reverse path congruent). For Node E to Node F there are three paths E-A-B-F, E-A-D-F, and E-C-D-F. The low-path-id algorithm uses path E-A-B-F and the high-path-id algorithm uses E-C-D-F. These paths are also disjoint and are reverse path congruent. Any nodes that are directly connected in this network have only one path between them (not shown for simplicity).
For Multicast paths the algorithms used are the same low-path-id or high-path-id but the tree is always a single tree using the root selected as described earlier (in this case Node A). Figure 3 shows the multicast paths for low-path-id and high-path-id algorithm.
All nodes in this network use one of these trees. The path for multicast to/from Node A is the same as unicast traffic to/from Node A for both low-path-id and high-path-id. However, the multicast path for other nodes is now different from the unicast paths for some destinations. For example, Node E to Node F is now different for high-path-id because the path must transit the root Node A. In addition, the Node E multicast path to C is E-A-C even though E has a direct path to Node C. A rule of thumb is that the node chosen to be root should be a well-connected node and have available resources. In this example, Node A and Node D are the best choices for root nodes.
The distribution of I-SIDs allows efficient pruning of the multicast single tree on a per I-SID basis because only MFIB entries between nodes on the single tree are populated. For example, if Nodes A, B and F share an I-SID and they use the low-path–id algorithm only those three nodes would have multicast traffic for that I-SID. If the high-path-id algorithm is used traffic from Nodes A and B must go through D to get to Node F.