Cisco Systems 3750E manual Bridge ID, Switch Priority, and Extended System ID, 18-4

Chapter 18 Configuring STP

Understanding Spanning-Tree Features

Only one outgoing port on the stack root switch is selected as the root port. The remaining switches in the stack become its designated switches (Switch 2 and Switch 3) as shown in Figure 18-1 on page 18-4.

•The shortest distance to the root switch is calculated for each switch based on the path cost.

•A designated switch for each LAN segment is selected. The designated switch incurs the lowest path cost when forwarding packets from that LAN to the root switch. The port through which the designated switch is attached to the LAN is called the designated port.

Figure 18-1	Spanning-Tree Port States in a Switch Stack
Catalyst 3750-E switch stack
	DP		Outgoing RP
		Switch 1
						DP
StackWise Plus	RP		BP	DP		Spanning-tree root
port
connections					Switch A
		Switch 2
	RP		DP	RP
		Switch 3			Switch B
RP = root port				159890
BP = blocked port
DP = designated port

All paths that are not needed to reach the root switch from anywhere in the switched network are placed in the spanning-tree blocking mode.

Bridge ID, Switch Priority, and Extended System ID

The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which controls the selection of the root switch. Because each VLAN is considered as a different logical bridge with PVST+ and rapid PVST+, the same switch must have a different bridge IDs for each configured VLAN. Each VLAN on the switch has a unique 8-byte bridge ID. The 2 most-significant bytes are used for the switch priority, and the remaining 6 bytes are derived from the switch MAC address.

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

18-4 OL-9775-02