Hitachi US7070447-001 manual Using Spanning Tree

Draft Level—Hitachi Confidential

HiSpeed Switch Troubleshooting Guide

VLAN 1

forwarding forwarding

Bridge 1 data looping Bridge 2

VLAN 1

Bridge 1 Bridge 2

				VLAN 2
forwarding			forwarding
		forwarding
	Bridge 3
				VLAN 3
		forwarding
				3011

blocking

VLAN 2

Bridge 3

VLAN 3

3012

Data looping Using Spanning Tree to eliminate ata looping

Figure 7-1. Using Spanning Tree

When Spanning Tree is implemented, redundant bridge ports are blocked and looping is eliminated, as shown in Figure 7-1.

The first step in forming a Spanning Tree is electing a root bridge. Initially, each bridge assumes it is the root bridge for the LAN. Bridge Protocol Data Unit (BPDU) packets are sent onto each of its ports.

The bridge broadcasts BPDU packets and listens for BPDU packets from other bridges. The bridge compares each packet transmitted with each packet received. The root bridge is finally elected based on the following:

❒Bridge with the Lowest Root ID. This is made up of a combination of the MAC address and bridge priority.

❒If the root IDs are equal, the bridge compares the costs (least number of hops). To ensure maximum performance, the bridge will calculate the shortest path possible to get to the root bridge.

❒If both the root IDs and the costs are equal, then the bridge compares the Transmitting Bridge ID's. The bridge will keep the frame with the lowest Transmitting Bridge ID.

If all of the parameters are equal, there is a final tie-breaker parameter known as the port identifier. Each port on a bridge has a port identifier, and the bridge will keep the BPDU frame with the numerically lower port ID.

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