EXP Computer S200 Setting the Root Bridge Spanning Tree, Determining Root Links Designated Links

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 any changes 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

80-00	08-00-3E-02-53-8F
Bridge Priority
		MAC Address

Figure 44. Example of Bridge ID and MAC Address

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 path 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 least 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 Figure 43, 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 and bridges 1, 4, and 5 are reporting 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 Cost 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.

90 Bridging