Priority; Determining Which Links Are Chosen: Setting Link Cost - HP ProCurve 7000dl Series Basic Management And Configuration Manual

Bridging—Transmitting Non-IP Traffic or Merging Two Networks
Configuring Spanning Tree
10-18
Determining Which Device Becomes Root: Setting the Router's

Priority

Spanning tree bridges elect the device with the lowest ID as the root. A bridge's
ID consists of its priority value plus its MAC address. By default, all interfaces
on the router have a priority of 32,768 (the standard default setting). Unless
you alter the priority setting, the switch with the lowest MAC address
becomes root.
Default settings, then, leave much to chance. A relatively unimportant device
may become root for an entire WAN. Your organization's IT staff should agree
on a hub router to become root for the bridged WAN. Lower this router's
priority with this global configuration mode command:
Syntax: spanning-tree priority <value>
Valid values are from 0 to 63535. Remember that lower values grant higher
priority.

Determining Which Links Are Chosen: Setting Link Cost

A BPDU includes the cost of the connection from the source of the BPDU to
the root device. Devices calculate this cost from the cost of all intervening
links. A device chooses which interface to make its root port according to
which interface receives the BPDU with the lowest cost.
A WAN router may have several connections with widely varying link speeds—
for example, a 100-Mbps connection to a switch and 3.0-Mbps connection
carried on two T1-carrier lines to a Frame Relay network. Assigning a higher
cost to the low-speed connection allows the router to take this discrepancy
into account when calculating best paths.
The Secure Router OS automatically calculates path cost from bandwidth, and
this setting is usually adequate. However, you may also want to consider the
monetary cost of link. If you are using a connection as a redundant link, you
should raise its cost to keep the router from choosing it as its primary link.
To change the cost of connection, move to the logical interface configuration
mode context for that link. Enter this command:
Syntax: spanning-tree path-cost <value>
Valid values are from 1 to 63,535. Remember to raise the cost for lower-speed
or redundant connections.