3Com PathBuilder S200 Series, Owner'S Manual

Page: 95 / 162

Bridging 91
T0008-16F Release 5.2M
Spanning Tree Protocol Entity (STPE)
Determining Path
Costs
Bridges use Path Cost to determine their Root Link. The range of Path Cost is 0 to
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:
• 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)
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.
Type of Network Speed STPE Path Cost
802.3 10 Mbps 10
802.5 4 Mbps 25
802.5 16 Mbps 6
serial 1.54 Mbps 65
serial 384 kbps 260
serial 56 kbps 1768
serial 19.2 kbps 3536