Bridge Priority

and the Root

Bridge

AT-GS950/16PS Switch Web Interface User’s Guide

The first task that bridges perform when a spanning tree protocol is activated on a network is the selection of a root bridge. A root bridge distributes network topology information to the other network bridges and is used by the other bridges to determine if there are redundant paths in the network.

A root bridge is selected by the bridge priority number, also referred to as the bridge identifier, and sometimes the bridge’s MAC address. The bridge with the lowest bridge priority number in the network is selected as the root bridge. If two or more bridges have the same lowest bridge priority number, the one with the lowest MAC address is designated as the root bridge.

You can change the bridge priority number in the AT-S112 Management software. You can designate which switch on your network as the root bridge by giving it the lowest bridge priority number. You may also consider which bridge should function as the backup root bridge in the event you need to take the primary root bridge off line and assign that bridge the second lowest bridge identifier number.

The bridge priority has a range 0 to 61440 in increments of 4096. To make this easier for you, the AT-S112 Management software divides the range into increments. You specify the increment that represents the desired bridge priority value. The range is divided into sixteen increments, as shown in Table 1.

Table 1 Bridge Priority Value Increments

Increment

Bridge

 

Increment

Bridge

Priority

 

Priority

 

 

 

 

 

 

 

 

 

 

 

 

 

0x0000

0

 

0x8000

32768

 

 

 

 

 

0x1000

4096

 

0x9000

36864

 

 

 

 

 

0x2000

8192

 

0xA000

40960

 

 

 

 

 

0x3000

12288

 

0xB000

45056

 

 

 

 

 

0x4000

16384

 

0xC000

49152

 

 

 

 

 

0x5000

20480

 

0xD000

53248

 

 

 

 

 

0x6000

24576

 

0xE000

57344

 

 

 

 

 

0x7000

28672

 

0xF000

61440

 

 

 

 

 

63

Page 63
Image 63
Allied Telesis AT-GS950/16PS, AT-S112 manual Bridge Priority Root, Increment Bridge Priority