3Com PathBuilder S200 Series Owner's Manual page 95

Determining Path Bridges use Path Cost to determine their Root Link. The range of Path Cost is 0 to
Costs
65535. The lower the path cost, the more likely this path will be used. Use This table
to determine the path costs for each type of link in your network.
For speeds not listed, interpolate to reasonable values. The valid range for path cost
is 0 to 65535 so that when values are determined, they should not be such that the
total path cost along any reasonable route adds up to more than 65535.
Some bridge manufacturers may list a different set of values for path cost increment.
It is important that the same rule be applied to all bridges involved in the spanning
tree calculation.
Returning to the mesh network in Figure 43, look at bridge B3: if the WAN lines are
all 19.2 kbps and the links are directly connected with a single SVC hop, then their
incremental cost for WAN links can be set to STPE Path Cost = 3536. Therefore, B3
will see messages from other bridge links resulting in the following cost to the root
bridge:
Based on these numbers, B3 determines B1 to be the root bridge, because B1's
bridge ID is lower than all reported root bridges (including B3's own bridge ID). B3
also designates bridge link 6 as the root link since it has the least cost to the root.
Bridging
T0008-16F
Type of Network
802.3
802.5
802.5
serial
serial
serial
serial
• From bridge 1 link 6: root is bridge 1, cost to root = 3536
• From bridge 4 link 8: root is bridge 1, cost to root = 7072 (3536+3536)
• From bridge 5 link 9: root is bridge 1, cost to root = 14154
(3536+10+3536+3536+3536)
Spanning Tree Protocol Entity (STPE)
Speed
10 Mbps
4 Mbps
16 Mbps
1.54 Mbps
384 kbps
56 kbps
19.2 kbps
STPE Path Cost
10
25
6
65
260
1768
3536
Release 5.2M
91