Spanning Tree Election Criteria
Spanning Tree builds paths out from a central point along the fastest
available links. It selects path according to the following criteria:
1. Lowest root bridge ID (BID)
2. Lowest path cost to the root
3. Lowest sender bridge ID
4. Lowest sender port ID (PID)
When reading the path selection criteria, remember the following:
■ Bridge ID—Bridge priority: Bridge MAC address.
■ Bridge priority—2-btye value, 0–65,535 (0–0xFFFF).
■ Default priority is 32,768 (0x8000).
■ Port ID—Port priority: port number.
■ Port priority—A 6-bit value, 0–63, default is 32.
■ Path cost—This is the cumulative value of the cost of each link
between the bridge and the root. Cost values were updated in 2000 and
you should see only new cost values, but both are given in the following
table (see Table 3-1). Old and new switches work together.