EXP Computer S200 80-00 08-00-3E-02-53-8F MAC Address, Bridge Priority

90 Bridging

Spanning Tree Protocol Entity (STPE)

Setting the Root

Bridge of the

Spanning Tree

The bridge with the lowest Bridge ID becomes the root bridge in a spanning tree

network.

The Bridge ID is made up of two parts: the Bridge Priority and the MAC address of

the LAN port. You modify these elements during bridge configuration from the

Bridge Priority parameter in the Bridge Parameters record and the MAC Address

parameter in the LAN Port record.

All bridges have the same default priority value (32768). So, without a ny c hanges to

this value, the MAC address of the LAN port determines the root bridge in a

network.

If you want to control which bridge becomes the root bridge, modify the Bridge

Priority value appropriately.

For example:

Bridge ID: equals (Bridge Priority Value in hex) + (MAC Address)

The default bridge priority value is 32768 (8000 in hex). So, a bridge with a MAC

address of 08-00-3E-02-53-8F and a default bridge priority value would have this

bridge ID:

Determining Root

Links and

Designated Links

A Root Link is the Bridge Link on a particular bridge that is the preferred path to the

Root Bridge.

A Designated Link is all other links that are part of the spanning tree.

In Automatic mode, the Root Link is determined by summing p ath costs from a

bridge to the Root Bridge. Path costs are configured in the Bridge Link Record. If

there are multiple paths to the Root Bridge, the bridge selects the route with the leas t

cost to the root as the preferred link (Root Link).

All other links associated with the bridge become Designated Links.

For example, in the network in Figure43, bridge 3 receives messages from bridges

1, 4, and 5 because these bridges are adjacent (directly connected by links). If the

cost of traversing any of the WAN links is equal a nd bridg es 1, 4, and 5 ar e repor ting

B1 as the root and that they know how to get to it, then bridge 3 will choose link 6 as

the preferred link to the root because this path will have the least cost to the root.

Note

The fewest number of links involved yields the lowest cost—the fact that the

link is directly attached to the root bridge is coincidental in this example.

The path cost to the root bridge has an influence on the spanning tree topology.

The bridge link parameter called STPE Path Co st is the parameter that sets the

incremental path cost to the root, should that bridge link be followed to the root

bridge. In general, the speed of the bridge link is the most important factor that

determines the path cost increment.

80-00 08-00-3E-02-53-8FMAC AddressBridge Priority