EXP Computer S200 manual Type of Network Speed Stpe Path Cost

Spanning Tree Protocol Entity (STPE)

Determining Path Bridges use Path Cost to determine their Root Link. The range of Path Cost is 0 to

Costs65535. 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.

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

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.

Bridging 91

T0008-16F Release 5.2M