Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 133

A path cost value is assigned to each port. The costs are accumulated to determine the total cost
to reach the root. According to the original specification in 802.1d, cost is calculated by dividing
1000 Mbps (1 gigabit per second) by the bandwidth (in megabits per second) of the segment
connected to the port. Using this formula, a 100 Mbps connection has a cost of 10 (1000 / 100
= 10). To accommodate higher speeds, such as GE, the IEEE adopted new values. Table 4-8
shows the revised path costs for STP.

STP Path Costs

Table 4-8

Bandwidth STP Cost Value

4 Mbps 250

10 Mbps 100

16 Mbps 62

45 Mbps 39

100 Mbps 19

155 Mbps 14

622 Mbps 6

1 Gbps 4

10 Gbps 2

Bridge Protocol Data Unit (BPDU)
Transparent bridges performing STP communicate by exchanging BPDUs. BPDU packets
contain information on ports, addresses, priorities and costs. Bridges do not forward received
BPDUs, instead the information generates new BPDUs.

The BPDU contains the following information:
•

Root BID —The BID of the bridge that the transmitting bridge believes to be the root
bridge.

•

Path Cost —The cost of the path to reach the root bridge. If the segment is attached to the
root bridge, it has a cost of 0. For example, if the data has to travel over three 10 Mbps
segments to reach the root bridge, the cost is 200 (100 + 100 + 0).

•

Sender BID —The BID of the bridge that sent this BPDU.

•

Port ID —The port ID on the bridge that sent this BPDU.

BPDU destination addresses use the bridge group multicast MAC address 01-80-C2-00-00-00.
These frames are copied by bridges but ignored by all other stations.

NOTE