RFC 6329

The Intermediate System to Intermediate System protocol (IS-IS), is defined in the IETF proposed standard RFC 6329, is used as the control plane for IEEE 802.1aq Shortest Path Bridging (SPB).^[1][2][3][4] 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.^[5]

SPB allows for shortest-path forwarding in an Ethernet mesh network context utilizing multiple equal cost paths. This permits SPB to support large Layer 2 topologies, with faster convergence, and vastly improved use of the mesh topology. Combined with this is single point provisioning for logical connectivity membership. IS-IS is therefore augmented with a small number of TLVs and sub-TLVs, and supports two Ethernet encapsulating data paths, 802.1ad Provider Bridges (PB) and 802.1ah Provider Backbone Bridges (PBB).

SPB is design 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 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 to 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. 802.1aq has been assigned the Network Layer Protocol ID (NLPID) value 0xC1, as per RFC 6328,^[6] and is used by SPB Bridges to indicate their ability to form adjacencies and operate as part of a 802.1aq domain. 802.1aq frames flow on adjacencies that advertise this NLPID in both directions, and nodes regard an adjacency that has not advertised in both directions as non-existent (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 Multiple Spanning Tree Configuration TLV which uses a common and compact encoding reused from 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/topology instance. This used in the formation of Multicast Destination Addresses (DAs) for frames originating from this node/instance.

There are multiple ECT algorithms defined for SPB; however, for the future, additional algorithms may be defined, 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 algorithm, 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. Where this sub-TLV is not present for an IS-IS adjacency, then that adjacency will not carry SPB traffic for the given topology instance.

There are multiple ECT algorithms defined for SPB; however, for the future, additional algorithms may be defined; similarly the SPB Adjacency Opaque Equal Cost Tree Algorithm TLV also occurs within the Multi-Topology Intermediate System TLV or the Extended IS Reachability TLV.

Service Information Extensions

The SPBM Service Identifier and Unicast Address TLV is used to introduce service group membership on the originating node and/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.

References