Allied Telesis AT-GS950/8 manual Path Costs and Port Costs, Increment Bridge Priority

AT-S82 Management Software User’s Guide

You can change the bridge priority number in the AT-S82 management software. You can designate which switch on your network you want as the root bridge by giving it the lowest bridge priority number. You might also consider which bridge should function as the backup root bridge in the event you need to take the primary root bridge offline, 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-S82 management software divides the range into increments. The valid range is of sixteen increments is shown in Table 3.

Table 3. Bridge Priority Value Increments

Increment	Bridge	Increment	Bridge
	Priority		Priority
0	0	8	32768
1	4096	9	36864
2	8192	10	40960
3	12288	11	45056
4	16384	12	49152
5	20480	13	53248
6	24576	14	57344
7	28672	15	61440

Path Costs and Port Costs

After the root bridge has been selected, the bridges must determine if the network contains redundant paths and, if one is found, they must select a preferred path while placing the redundant paths in a backup or blocking state.

Where there is only one path between a bridge and the root bridge, the bridge is referred to as the designated bridge and the port through which the bridge is communicating with the root bridge is referred to as the root port.

If redundant paths exist, the bridges that are a part of the paths must determine which path will be the primary, active path, and which path(s) will be placed in the standby, blocking mode. This is accomplished by an determination of path costs. The path offering the lowest cost to the root bridge becomes the primary path and all other redundant paths are placed into blocking state.

Path cost is determined through an evaluation of port costs. Every port on

63