IEEE 802.1aq is an amendment to the IEEE 802.1Q networking standard which adds support for Shortest Path Bridging (SPB). This technology is intended to simplify the creation and configuration of Ethernet networks while enabling multipath routing.
SPB is designed to replace the older Spanning Tree Protocols: IEEE 802.1D STP, IEEE 802.1w RSTP, and IEEE 802.1s MSTP. These block any redundant paths that can result in a switching loop, whereas SPB allows all paths to be active with multiple equal-cost paths, provides much larger topologies, supports faster convergence times, and improves the efficiency by allowing traffic to load share across all paths of a mesh network. It is designed to preserve the plug-and-play nature that established Ethernet as the de facto protocol at layer 2.
The technology provides VLANs on native Ethernet infrastructures using a link-state protocol to advertise both topology and VLAN membership. Packets are encapsulated at the edge either in MAC-in-MAC per IEEE 802.1ah or tagged per IEEE 802.1Q or IEEE 802.1ad and transported only to other members of VLAN. Unicast, multicast, and broadcast are supported and all routing is on symmetric shortest paths.
The control plane is based on the Intermediate System to Intermediate System (IS-IS) routing protocol, leveraging a small number of extensions defined in .
History On 4 March 2006 the working group posted 802.1aq draft 0.1. In March 2012 the IEEE approved the 802.1aq standard.
In May 2013, the first public multivendor interoperability was demonstrated as SPB served as the backbone for Interop 2013 in Las Vegas. In 2013 and 2014 SPB was used to build the InteropNet backbone with only one-tenth the resources of prior years. During Interop 2014 SPB was used as the backbone protocol which can enable software-defined networking (SDN) functionalities.
The 2014 Winter Olympics were the first "fabric-enabled" Games using SPB "IEEE 802.1aq" technology. During the games this fabric network could handle up to 54 Tbit/s of traffic.
Associated protocols IEEE 802.1Q-2014 - Bridges and Bridged Networks - This standard incorporates Shortest Path Bridging (IEEE 802.1aq) with the following: IEEE Std 802.1Q-2011, IEEE Std 802.1Qbe-2011, IEEE Std 802.1Qbc-2011, IEEE Std 802.1Qbb-2011, IEEE Std 802.1Qaz-2011, IEEE Std 802.1Qbf-2011, IEEE Std 802.1Qbg-2012, IEEE Std 802.1Q-2011/Cor 2–2012, and IEEE Std 802.1Qbp-2014, and much functionality previously specified in 802.1D. IEEE 802.1ag - Connectivity Fault Management (CFM) IEEE 802.1Qbp - Equal Cost Multiple Paths in Shortest Path Bridging IEEE P802.1Qcj - Automatic Attachment to Provider Backbone Bridging (PBB) services RFC 6329 - IS-IS Extensions Supporting IEEE 802.1aq Shortest Path Bridging
RFC 6329 The Intermediate System to Intermediate System (IS-IS) protocol, as defined in the IETF proposed standard , is used as the control plane for SPB. SPB requires no state machine or other substantive changes to IS-IS, and simply requires a new Network Layer Protocol Identifier (NLPID) and set of TLVs.
SPB allows for shortest-path forwarding in a mesh-connected Ethernet network utilizing multiple equal-cost paths. This permits SPB to support large Layer 2 topologies, with faster convergence, and improved use of the mesh topology when compared to networks configured with Spanning Tree Protocol. SPB augments IS-IS with a small number of TLVs and sub-TLVs, and supports two Ethernet-encapsulating data paths, IEEE 802.1ad provider bridges (PB) and IEEE 802.1ah Provider Backbone Bridges (PBB).
SPB is designed to run in parallel with other network-layer protocols such as IPv4 and IPv6. Standards mandate that the failure of two nodes to establish an SPB adjacency will not have a collateral impact, such as the rejection of an adjacency for other network-layer protocols (e.g., OSPF).
Protocol extensions The IS-IS extensions defined in RFC 6329 that deliver standardized support for 802.1aq SPB are: IS-IS Hello (IIH) Protocol Extensions Node Information Extensions Adjacency Information Extensions Service Information Extensions
IS-IS Hello (IIH) protocol extensions 802.1aq has been designed to operate in parallel with other network layer protocols such as IPv4 and IPv6; therefore, failure of two nodes to establish an SPB adjacency will not cause network layer protocols to also reject an adjacency. RFC 6328 assigns 802.1aq the Network Layer Protocol ID (NLPID) value 0xC1. This NLPID is used by SPB Bridges to indicate their ability to form adjacencies and operate as part of an 802.1aq domain. 802.1aq frames flow on adjacencies that advertise this NLPID in both directions, and nodes regard an adjacency that has not been advertised in both directions as non-existent (with infinite link metric). 802.1aq augments the normal IIH PDU with three new TLVs, which, like all other SPB TLVs, travel within Multi-Topology TLVs, therefore allowing multiple logical instances of SPB within a single IS-IS protocol instance.
SPB can use many VIDs, agreeing on which VIDs are used for which purposes. The IIH PDUs carry a digest of all the used VIDs, referred to as the , which uses a common and compact encoding reused from IEEE 802.1Q.
For the purposes of loop prevention, SPB neighbors may also support a mechanism to verify that the contents of their topology databases are synchronized. Exchanging digests of SPB topology information, using the optional SPB-Digest sub-TLV, allows nodes to compare information and take specific action where a mismatch in topology is indicated.
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.
Node information extensions All SPB nodal information extensions travel within a new Multi-Topology (MT) capability TLV. There can be one or many MT-Capability TLVs present, depending on the amount of information that needs to be carried.
The SPB Instance sub-TLV gives the Shortest Path Source ID (SPSourceID) for this node or topology instance. This is used in the formation of multicast destination addresses (DAs) for frames originating from this node or instance.
There are multiple ECT algorithms defined for SPB and additional algorithms may be defined in the future, including but not limited to ECMP- or hash-based behaviors and (*,G) Multicast trees. These algorithms will use this optional TLV to define new algorithm parametric data. For tie-breaking parameters, there are two broad classes of algorithms: one that uses nodal data to break ties and one that uses link data to break ties. The SPB Instance Opaque Equal cost Tree Algorithm TLV is used to associate opaque tie-breaking data with a node.
Adjacency information extensions The SPB Link Metric sub-TLV occurs within the Multi-Topology Intermediate System Neighbor TLV or within the Extended IS Reachability TLV. SPB Adjacency Opaque Equal Cost Tree Algorithm TLV also occurs within the Multi-Topology Intermediate System TLV or the Extended IS Reachability TLV. Where this sub-TLV is not present for an IS-IS adjacency, that adjacency will not carry SPB traffic for the given topology instance.
Service information extensions The SPBM Service Identifier and Unicast Address TLV is used to introduce service group membership on the originating node or to advertise an additional B-MAC unicast address present on, or reachable by the node. The SPBV MAC Address TLV is the IS-IS sub-TLV used for advertisement of group MAC addresses in SPBV mode.
Benefits
Shortest Path Bridging-VID (SPBV) and Shortest Path Bridging-MAC (SPBM) are two operating modes of 802.1aq. Both inherit key benefits of link state routing: the ability to use all available physical connectivity, because loop avoidance uses a control plane with a global view of network topology fast restoration of connectivity after failure, again because of link state routing's global view of network topology under failure, the property that only directly affected traffic is impacted during restoration rapid restoration of broadcast and multicast connectivity, because IS-IS floods all of the required information in the SPB extensions to IS-IS, thereby allowing unicast and multicast connectivity to be installed in parallel, with no need for a second phase signaling process to run over the converged unicast topology to compute and install multicast trees.
SPBM offers emulation of a transparent Ethernet LAN segment. It implements VLANs with scoped multicast trees, which means no egress discard of broadcast, unknown-unicast and multicast traffic, a feature common to approaches that use a small number of shared trees; hence the network does not simply degrade with size as the percentage of frames discarded goes up.
The carrier-space equivalent of this application is the delivery of Ethernet VPN services to Enterprises over common carrier infrastructure. The required attributes are fundamentally the same; complete transparency for customer Ethernet services (both point-to-point and LAN), and complete isolation between one customer's traffic and that of all other customers.
A further consequence of SPBM's transparency in both data plane and control plane is that it delivers the MEF 6.1 service set. It also provides the carrier with the toolkit to support geo-redundant broadband backhaul; in these applications, many DSLAMs or other access equipment must be backhauled to multiple Broadband Remote Access Server (BRAS) sites, with application-determined binding of sessions to a BRAS. However, DSLAMs must not be allowed to communicate with each other, because carriers then lose the ability to control peer-to-peer connectivity. MEF E-TREE does just this, and further provides an efficient multicast fabric for the distribution of IPTV.
SPBM offers both an ideal multicast replication model, where packets are replicated only at fork points in the shortest path tree that connects members, and also the less state-intensive head-end replication model, where, in essence, serial unicast packets are sent to all other members along the same shortest path first tree. These two models are selected by specifying properties of the service at the edge, which affect the transit node decisions on multicast state installation. This allows for a trade-off to be made between optimum transit replication points (with their larger state costs) vs. reduced core state (but much more traffic) of the head-end replication model. These selections can be different for different members of the same Individual Service ID (I-SID), allowing different trade-offs to be made for different members.
Figure 5 below is a quick way to understand what SPBM is doing on the scale of the entire network. 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. Head-end replication is not shown as it is trivial and simply uses the existing unicast forwarding information base (FIB) to forward copies serially to the known other receivers.
Operations and management 802.1aq builds on all existing Ethernet operations, administration and management (OA&M). 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.1731ITU-T Recommendation Y.1731 OAM functions and mechanisms for Ethernet based networks operate unchanged on an 802.1aq network.
High level