3com 3800 Manual Do Utilizador

C

HAPTER

 7: S

PANNING

 T

REE

 P

ROTOCOL

 (STP)

The Root Bridge generates BPDUs on all ports at a regular interval 
known as the Hello Time. All other bridges in the network have a Root 
Port. This is the port that costs the least in getting to the Root Bridge, 
and it is used for receiving the BPDUs initiated by the Root Bridge.

After all bridges on the network have determined the configuration of 
their ports, each bridge only forwards traffic between the Root Port 
and the ports that are the Designated Bridge Ports for each network 
segment to which they are attached. All other ports are blocked, which 
means that they are prevented from forwarding traffic.

In the event of a network failure (such as a segment going down) the 
STP system reconfigures the network to adjust for the changes. If the 
topology of the network changes, the Root Bridge sends out an SNMP 
trap.

The Switch 3800 can be partitioned into multiple virtual bridges. Each 
virtual bridge can run an independent spanning tree instance. Each 
spanning tree instance is called a Spanning Tree Domain (STPD). Each 
STPD has its own Root Bridge and active path. Once the STPD is 
created, one or more VLANs can be assigned to it.

A port can belong to only one STPD. If a port is a member of multiple 
VLANs, then all those VLANs must belong to the same STPD.

The key points to remember when configuring VLANs and STP are the 
following:

■

Each VLAN forms an independent broadcast domain.

■

STP blocks paths to create a loop-free environment.

■

When STP blocks a path, no data can be transmitted or received on 
the blocked port.

SW3800.BK  Page 4  Tuesday, May 5, 1998  5:20 PM