IS-IS

SPBM eliminates the need for multiple overlay protocols in the core of the network by reducing the core to a single Ethernet-based, link-state protocol (IS-IS). IS-IS provides virtualization services using a pure Ethernet technology base. SPBM also uses IS-IS to discover and advertise the network topology, which enables it to compute the shortest path to all nodes in the SPBM network.

IS-IS is a link-state, interior gateway protocol that was developed for the International Organization for Standardization (ISO). ISO terminology refers to routers as Intermediate Systems (IS), hence the name Intermediate System-to-Intermediate System (IS-IS).

To provide a loop-free network and to learn and distribute network information, SPBM uses the Intermediate-System-to-Intermediate-System (IS-IS) link state routing protocol. IS-IS is designed to find the shortest path from any one destination to any other in a dynamic fashion. IS-IS creates any-to-any connectivity in a network in an optimized, loop-free manner, without the long convergence delay experienced with the Spanning Tree Protocol. IS-IS does not block ports from use, but rather employs a specific path. As such, all links are available for use.

IS-IS dynamically learns the topology of a network and constructs unicast and multicast mesh connectivity. IS-IS parallel adjacency support allows you to configure multiple IS-IS links between the two nodes. Each node in the network calculates a shortest-path tree to every other network node based on System-IDs (B-MAC addresses). Only one adjacency with the shortest path is selected as an active adjacency.

Note

Only an active interface with an active adjacency is added into local SPF calculations. This mechanism ensures the local node selects the shortest path and has the same view as the rest of the SPB network.

In the SPBM environment for Layer 2 VSNs, IS-IS carries only pure Layer 2 information with no requirement for an underlying IP control plane or forwarding path. IS-IS runs directly over Layer 2.

Note

SPBM carries Layer 3 information for Layer 3 VSNs.

In SPBM networks, IS-IS performs the following functions:

• Discovers the network topology

• Builds shortest path trees between the network nodes:

  • Forwards unicast traffic

  • Determines the forwarding table for multicast traffic

• Communicates network information in the control plane:

  • Service Instance Identifier (I-SID) information

SPBM can distribute I-SID service information to all SPBM nodes, as the I-SIDs are created. SPBM includes I-SID information in the IS-IS Link State protocol data units (PDUs). When a new service instance is provisioned on a node, its membership is flooded throughout the topology using an IS-IS advertisement.