The STP Root Bridge

Reference point One root per VLAN Maintains topology Propagates timers

STP uses the concepts of root bridge, root ports, and designated ports to establish a loop-free path through the network. The first step in creating the loop-free spanning tree is to elect a root bridge. The root bridge is the reference point that all switches use to establish forwarding paths that will avoid loops in the Layer 2 network.

The main information to be concerned with is the root ID (bridge that the transmitting bridge thinks is the root), BID, and cost (which is the cost to the root bridge). The STP topology is considered converged after a root bridge has been selected and each bridge has selected its root port, designated bridge, and the ports that will participate in the STP topology. STP uses these configuration messages (BPDUs) as it transitions port states to achieve convergence.

Spanning tree elects one bridge on the LAN to be the master bridge. This bridge is called the root bridge. The root bridge is special because all the path calculation through the network is based on the root. The bridge is elected based on the BID, which consists of a 2-byte Priority field plus a 6-byte MAC address. In spanning tree, lower BID values are preferred. In a default configuration, the Priority field is set at 32768.

Because the default Priority field is the same for all the bridges, the root selection is based on the lowest MAC address. One method of selecting a specific bridge to be the root is to manually alter the Priority field to a lower value. Regardless of what the MAC address is, the Priority field decides which bridge is going to be the root, assuming that all bridges do not have the same priority value.

3-10 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

When a topology change occurs as a result of switch link state changes, the root will send messages throughout the tree regarding the topology change. This allows the content addressable memory (CAM) tables to adjust and to provide for a new path that may be used toward end host devices.

Timer information is also sent by the root bridge to nonroot bridges, informing them of the intervals to use as the ports transition through the spanning tree port states.

The root bridge maintains the stability of the forwarding paths between all switches for a single STP instance. A spanning tree instance is a configuration in which all switches exchanging BPDUs and participating in spanning tree negotiation are associated with a single root. If this is done for all VLANs, it is called a Common Spanning Tree (CST) instance. There is also a Per VLAN Spanning Tree (PVST) implementation that provides one instance, and therefore one root bridge, for each VLAN.

© 2006 Cisco Systems, Inc. Implementing Spanning Tree 3-11

Continue reading here: Spanning Tree Port States

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