3Com SuperStack II - STP Configurations

How STP Works 199
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Bridge A has the lowest Bridge Identifier in the network, and has
therefore been selected as the Root Bridge.
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Because Bridge A is the Root Bridge, it is also the Designated Bridge
for LAN segment A. Port 1 on Bridge A is therefore selected as the
Designated Bridge Port for segment A.
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Port 1 of Bridges B, C, X and Y have been defined as a Root Ports
because they are the nearest to the Root Bridge.
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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 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 B.
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Bridge C has been selected as the Designated Bridge for LAN segment
B, because it offers the lowest Root Path Cost for segment C — the
route through Bridges C and B costs 200 (C–B=100, B–A=100), the
route through Bridges Y and B costs 300 (C–B=200, B–A=100). Port 2
on Bridge C is therefore selected as the Designated Bridge Port for
segment C.
STP Configurations
Figure 49 (overleaf) shows three possible STP configurations using
SuperStack II Switch units.
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Configuration 1 — Redundancy for Backbone Link
In this configuration, a Switch 1100 and a Switch 3300 both have STP
enabled and are connected by two links. STP discovers a duplicate
path and disables one of the links. If the enabled link breaks, the
disabled link becomes re-enabled, therefore maintaining connectivity.
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Configuration 2 — Redundancy through Meshed Backbone
In this configuration, four Switch 3300 units are connected such that
there are multiple paths between each one. STP discovers the
duplicate paths and disables two of the links. If an enabled link breaks,
one of the disabled links becomes re-enabled, therefore maintaining
connectivity.
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Configuration 3 — Redundancy for Cabling Error
In this configuration, a Switch 1100 has STP enabled and is
accidentally connected to a hub using two links. STP discovers a
duplicate path and disables one of the links, therefore avoiding a loop.