How STP Works 199 Bridge A has the lowest Bridge Identifier in the network, and hastherefore been selected as the Root Bridge. Because Bridge A is the Root Bridge, it is also the Designated Bridgefor LAN segment A. Port 1 on Bridge A is therefore selected as theDesignated Bridge Port for segment A. Port 1 of Bridges B, C, X and Y have been defined as a Root Portsbecause they are the nearest to the Root Bridge. Bridges B and X offer the same Root Path Cost for LAN segment B,however, Bridge B has been selected as the Designated Bridge for thesegment because it has a lower Bridge Identifier. Port 2 on Bridge B istherefore selected as the Designated Bridge Port for LAN segment B. Bridge C has been selected as the Designated Bridge for LAN segmentB, because it offers the lowest Root Path Cost for segment C — theroute through Bridges C and B costs 200 (C–B=100, B–A=100), theroute through Bridges Y and B costs 300 (C–B=200, B–A=100). Port 2on Bridge C is therefore selected as the Designated Bridge Port forsegment C.STP Configurations Figure 49 (overleaf) shows three possible STP configurations usingSuperStack II Switch units. Configuration 1 — Redundancy for Backbone LinkIn this configuration, a Switch 1100 and a Switch 3300 both have STPenabled and are connected by two links. STP discovers a duplicatepath and disables one of the links. If the enabled link breaks, thedisabled link becomes re-enabled, therefore maintaining connectivity. Configuration 2 — Redundancy through Meshed BackboneIn this configuration, four Switch 3300 units are connected such thatthere are multiple paths between each one. STP discovers theduplicate paths and disables two of the links. If an enabled link breaks,one of the disabled links becomes re-enabled, therefore maintainingconnectivity. Configuration 3 — Redundancy for Cabling ErrorIn this configuration, a Switch 1100 has STP enabled and isaccidentally connected to a hub using two links. STP discovers aduplicate path and disables one of the links, therefore avoiding a loop.
