3Com 3C17300A Implementation Guide - Page 46
STP Configurations, Configuration 1 - Redundancy for Backbone Link
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46 CHAPTER 4: USING RESILIENCE FEATURES ■ Because Bridge A is the Root Bridge, it is also the Designated Bridge for LAN segment 1. Port 1 on Bridge A is therefore selected as the Designated Bridge Port for LAN Segment 1. ■ Port 1 of Bridges B, C, X and Y have been defined as Root Ports because they are the nearest to the Root Bridge and therefore have the most efficient path. ■ Bridges B and X offer the same Root Path Cost for LAN segment 2, however, Bridge B has been selected as the Designated Bridge for the segment because it has a lower Bridge Identifier. Port 2 on Bridge B is therefore selected as the Designated Bridge Port for LAN Segment 2. ■ Bridge C has been selected as the Designated Bridge for LAN segment 3, because it offers the lowest Root Path Cost for LAN Segment 3: ■ the route through Bridges C and B costs 200 (C to B=100, B to A=100) ■ the route through Bridges Y and B costs 300 (Y to B=200, B to A=100). Port 2 on Bridge C is therefore selected as the Designated Bridge Port for LAN Segment 3. STP Configurations Figure 11 shows three possible STP configurations using SuperStack 3 Switch units. ■ Configuration 1 - Redundancy for Backbone Link In this configuration, the Switches both have STP enabled and are connected by two links. STP discovers a duplicate path and blocks one of the links. If the enabled link breaks, the disabled link becomes re-enabled, therefore maintaining connectivity. ■ Configuration 2 - Redundancy through Meshed Backbone In this configuration, four Switch units are connected in a way that creates multiple paths between each one. STP discovers the duplicate paths and blocks two of the links. If an enabled link breaks, one of the disabled links becomes re-enabled, therefore maintaining connectivity. ■ Configuration 3 - Redundancy for Cabling Error In this configuration, a Switch has STP enabled and is accidentally connected to a hub using two links. STP discovers a duplicate path and blocks one of the links, therefore avoiding a loop.