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HiSpeed Switch Troubleshooting Guide
7-2
US7070447-001, Rev 01
Draft Level—Hitachi Confidential
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.
forwarding
forwarding
data looping
forwarding
forwarding
forwarding
forwarding
Bridge 1
Bridge 3
Bridge 2
VLAN 1
VLAN 2
VLAN 3
3011
not forwarding
Bridge 1
Bridge 3
Bridge 2
VLAN 1
VLAN 2
VLAN 3
3012
blocking
Data looping Using Spanning Tree to eliminate ata looping