Cisco Systems 3750E manual Spanning-Tree Topology and BPDUs, 18-3

Chapter 18 Configuring STP

Understanding Spanning-Tree Features

Spanning-Tree Topology and BPDUs

The stable, active spanning-tree topology of a switched network is controlled by these elements:

•The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch. In a switch stack, all switches use the same bridge ID for a given spanning-tree instance.

•The spanning-tree path cost to the root switch.

•The port identifier (port priority and MAC address) associated with each Layer 2 interface.

When the switches in a network are powered up, each functions as the root switch. Each switch sends a configuration BPDU through all of its ports. The BPDUs communicate and compute the spanning-tree topology. Each configuration BPDU contains this information:

•The unique bridge ID of the switch that the sending switch identifies as the root switch

•The spanning-tree path cost to the root

•The bridge ID of the sending switch

•Message age

•The identifier of the sending interface

•Values for the hello, forward delay, and max-age protocol timers

When a switch receives a configuration BPDU that contains superior information (lower bridge ID, lower path cost, and so forth), it stores the information for that port. If this BPDU is received on the root port of the switch, the switch also forwards it with an updated message to all attached LANs for which it is the designated switch.

If a switch receives a configuration BPDU that contains inferior information to that currently stored for that port, it discards the BPDU. If the switch is a designated switch for the LAN from which the inferior BPDU was received, it sends that LAN a BPDU containing the up-to-date information stored for that port. In this way, inferior information is discarded, and superior information is propagated on the network.

A BPDU exchange results in these actions:

•One switch in the network is elected as the root switch (the logical center of the spanning-tree topology in a switched network). In a switch stack, one stack member is elected as the stack root switch. The stack root switch contains the outgoing root port (Switch 1), as shown in Figure 18-1 on page 18-4.

For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is elected as the root switch. If all switches are configured with the default priority (32768), the switch with the lowest MAC address in the VLAN becomes the root switch. The switch priority value occupies the most significant bits of the bridge ID, as shown in Table 18-1 on page 18-5.

•A root port is selected for each switch (except the root switch). This port provides the best path (lowest cost) when the switch forwards packets to the root switch.

When selecting the root port on a switch stack, spanning tree follows this sequence:

–Selects the lowest root bridge ID

–Selects the lowest path cost to the root switch

–Selects the lowest designated bridge ID

–Selects the lowest designated path cost

–Selects the lowest port ID

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 18-3