Using Shortest Path Bridging (SPB) in Your Network

Shortest Path Bridging (SPB), IEEE 802.1aq, is a protocol that provides data traffic a shortest cost path between any pair of switches in the SPB network. SPB features dynamic route calculation in a loop-free Layer-2 network and fast convergence time using IS-IS. The S- K- and 7100-Series supports Shortest Path Bridging VLAN (SPBV).

SPB is administratively enabled by default, but requires that Spanning Tree be configured with the Spanning Tree version set to SPT, and the MST configuration name must be the same on all devices for a given SPB region. You can configure multiple SPB regions by assuring that each device within a given SPB region has the same MST configuration name. Spanning Tree defaults to version 3 (MSTP).

SPB uses IS-IS link state to populate the network topology information and calculate the Shortest Path Trees (SPTs). No IS-IS configuration is required. IS-IS is automatically enabled with a default NET as part of SPB region setup. The SPB capable bridge provides user access to an SPB region. A bridge joins an SPB region by forming an adjacency with a neighboring bridge through the exchange of IS-IS PDUs.

The base-VLAN is the customer VLAN that ingresses the SPBV network. It is used to manage operations in the SPT and provides access to the SPBV network. The Shortest Path VID (SPVID) is a VLAN used to identify a base-VLAN and SPT within the SPB network. Once a packet ingresses the SPB network, the SPVID is used. When the packet egresses the SPB network, the SPVID is translated back to the base-VLAN.

There are two types of ports in an SPBV region. Packets ingress and egress the SPB region on boundary ports. The boundary port is a member of the base-VLAN. Internal ports face into the SPB region and are members of the SPVID. An SPVID pool is administratively configured. The VLANs in the SPVID pool can only be used as SPVIDs. All SPB region ports must be administratively enabled for SPB.

Each base-VLAN at the boundary port is administratively assigned one of 16 Equal Cost Tree (ECT) algorithms as defined in IEEE 802.1Qaq. Each base-VLAN within the same SPB region must be assigned the same ECT algorithm. When traffic ingresses the SPB region on the base-VLAN, the packet VLAN (base-VLAN) is translated to an SPVID. By default this translation is dynamic, but you can optionally administratively set the base-VLAN to SPVID mapping.

Each VLAN on a bridge is associated with an SPVID whose tree is rooted at that bridge. This unique mapping is enforced by ISIS. As the traffic passes through the SPBV network, the SPVID is used. The base-VLAN and SPVID follow normal MAC and FID learning by making use of shared VLAN learning, but it‘s FID will be defined by the mapped base-VLAN. Since the SPVID is unique to a device and base-VLAN, the SPVID defines the source bridge of traffic received within the SPBV region. The SPVID pool must be a size equal to or greater than the number of base-VLANs times the number of nodes in the SPB region that switch base-VLAN traffic. The ECT determines the next switch in the path through the SPB network. When the packet egresses the SPB network at a boundary port, the SPVID is translated back to the appropriate VLAN before being forwarded.

SPB Overview shows an example where a user is trying to access server C in the Data Center through use of a spanning tree switched network. Spanning tree determines bridge A is the root of the CIST, and is blocking ports on bridges C, D, and E. The red line would be a normal MSTP switched traffic path, which makes an extra hop through router A. The shortest path is skipped due to the blocked port. The SPB network comprises a set of trees, each tree rooted at a switch where traffic ingresses the region. No ports are blocked and ISIS will create a shortest path through B and E to server C on the blue line.

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SPB Overview
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SPB provides benefits in addition to lower latency due to fewer hops. Use of IS-IS allows for quicker network recovery times than traditional Spanning Tree. SPB also provides for a more even distribution of traffic so switches, such as bridge A in SPB Overview, do not become bottlenecks. SPB makes use of Equal Cost Trees (ECTs) to further refine traffic distribution.

Traffic can be mapped to access the SPBV backplane with different base-VLANs. A base-VLAN with one ECT algorithm may make a different path choice than another base-VLAN with a different ECT algorithm when faced with paths of equal cost. A network can be set up where traffic received on specific VLANs outside the SPBV network will all map to a single base-VLAN and will use the same ECT path, for example the blue line in SPBV Using Equal Cost Trees. Other VLANs can be mapped to a different base-VLAN and use a different ECT path, as indicated by the red line. All devices that are associated with a particular base-VLAN must be configured with the same ECT algorithm.

SPBV provides shortest paths without requiring changes in MTU size because it performs VID translation rather than adding its own header. It also fully supports congestion notification.

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SPBV Using Equal Cost Trees
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