IEEE 802.1aq
The IIH PDUs carry a digest of all the used VIDs, referred to as the Multiple Spanning Tree Configuration TLV which uses a common and compact encoding reused from IEEE 802.1Q.
Finally, SPB needs to know which shortest path tree (SPT) sets are being used by which VIDs, and this is carried in the Base VLAN Identifiers TLV.
Both inherit key benefits of link state routing: SPBM offers emulation of a transparent Ethernet LAN segment.
These two models are selected by specifying properties of the service at the edge which affect the transit node decisions on multicast state installation.
Figure 5 shows how a 7-member E-LAN is created from the edge membership information and the deterministic distributed calculation of per source, per service trees with transit replication.
Since 802.1aq ensures that its unicast and multicast packets for a given virtual LAN (VLAN) follow the same forward and reverse path and use completely standard 802 encapsulations, all the methods of IEEE 802.1ag and Y.1731[27] operate unchanged on an 802.1aq network.
Depending on the topology, several different equal-cost multi-path trees are possible and SPB supports multiple algorithms per IS-IS instance.
Each node computes the Ethernet-compliant forwarding behavior independently based on a normally synchronized common view of the network and UNI ports.
One (SPBM) is intended where complete isolation of many separate instances of client LANs and their associated device MAC addresses is desired, and it therefore uses a full encapsulation (MAC-in-MAC a.k.a.
The other (SPBV) is intended where such isolation of client device MAC addresses is not necessary, and it reuses only the existing VLAN tag on participating NNI links.
Shortest Path Bridging-MAC (SPBM) reuses the PBB data plane which does not require that the Backbone Core Bridges (BCB) learn encapsulated client addresses.
Since only bridges on a shortest path between participating logical ports create forwarding database (FDB) state the multicast makes the efficient use of network resources.
The only difference compared with classical Ethernet is that reverse learning is disabled for participating bridge backbone media access control (B-MAC) addresses and is replaced with an ingress check and discard (when the frame arrives on an incoming interface from an unexpected source).
Properly implemented an IEEE 802.1aq network can support up to 1000 participating bridges and provide tens of thousands of layer 2 E-LAN services to Ethernet devices.
As new members come and go, the IS-IS protocol will advertise the I-SID membership changes and the computations will grow or shrink the trees in the participating node network as necessary to maintain the efficient multicast property for that service.
Since no Ethernet addresses are advertised or known by this protocol, there is no re-learning required by the SPBM core and its learned encapsulations are unaffected by a transit node or link failure.
A special attribute of SPBM is its ability to rebuild multicast trees in a similar time to unicast convergence, because it substitutes computation for signaling.
When an SPBM bridge has performed the computations on a topology database, it knows whether it is on the shortest path between a root and one or more leaves of the SPT and can install state accordingly.
This very efficient mechanism uses exchange of a single digest of link state covering the entire network view, and does not need agreement on each path to each root individually.
The last of these animated gifs, shown in Figure 7, demonstrates source destination ECT paths using all 16 of the standard algorithms currently defined.
ECMT in an IEEE 802.1aq network is more predictable than with internet protocol (IP) or multiprotocol label switching (MPLS) because of symmetry between the forward and reverse paths.
Such computations are used by time distribution protocols such as IEEE 1588 for frequency and time-of-day synchronization as required between precision clock sources and wireless base stations.
These are composites of screen captures of an 802.1aq network emulator and show the source in purple, the destination in yellow, and then all the computed and available shortest paths in pink.
The animations show three different networks and a variety of source and destination pairs which continually change to help visualize what is happening.
It is expected that other standards groups or vendors will produce variations on the currently defined algorithms with behaviors suited for different networks styles.
The only real limiting factors are the FIB table sizes and computational power of the individual devices both of which are growing yearly in leaps and bounds.
An implementation of 802.1aq will first modify the IS-IS hellos to include an NLPID (network layer protocol identifier) of 0xC01 in their Protocols-Supported type–length–value (TLV) (type 129) which has been reserved for 802.1aq.
A reference to a paper presented at the IEEE which gives a much faster algorithm that drastically reduces the number of outer iterations required is given below.
In general though even the exhaustive algorithm above is more than able to handle several hundred node networks in a few tens of milliseconds on the 1 GHz or greater common CPUs when carefully crafted.
Additionally, Extreme Networks is supporting an IETF Internet Draft Draft that defines a means of automatically extended SPBM-based services to end-devices via conventional Ethernet Switches, leveraging an IEEE 802.1AB LLDP-based communications protocol; this capability - marketing "Fabric Attach" technology - allows for the automatic attachment of end-devices, and includes dynamic configuration of VLAN/I-SID (VSN) mappings.
