Allied Telesis AT-S79 manual Port Speed Port Cost, Port Speed No. of Ports Port Cost Trunk

Chapter 9: Rapid Spanning Tree Protocol (RSTP)

Table 3 lists the RSTP port costs with Auto-Detect.

Table 3. RSTP Auto-Detect Port Costs

Port Speed	Port Cost
10 Mbps	2,000,000
100 Mbps	200,000
1000 Mbps	20,000

Table 4 lists the RSTP port costs with Auto-Detect when the port is part of a port trunk.

Table 4. RSTP Auto-Detect Port Trunk Costs

Port Speed	No. of Ports/	Port Cost
	Trunk
10/100/1000	2	10,000
10/100/1000	3	6,666
10/100/1000	4	5,000
10/100/1000	5	4,000
10/100/1000	6	3,333
10/100/1000	7	2,857
10/100/1000	8	2,500

You can override Auto-Detect and set the port cost manually. However, you must assign the same port cost to all ports that are members of a trunk.

Port Priority

If two paths have the same port cost, the bridges must select a preferred path. In some instances this can involve the use of the port priority parameter. This parameter is used as a tie breaker when two paths have the same cost.

The range for port priority, in hexadecimal format, is 0 to 240, with 240 being the highest priority. As with bridge priority, this range is broken into multiples of 16. To select a port priority for a port, you enter the desired value.

110 Section I: Using the Menus Interface