Stp Configurations - 3Com 3C17300-US Implementation Manual

46 C 4: U
HAPTER SING

STP Configurations

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ESILIENCE EATURES
Because Bridge A is the Root Bridge, it is also the Designated Bridge
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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
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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,
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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
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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
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A=100)
the route through Bridges Y and B costs 300 (Y to B=200, B to
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A=100).
Port 2 on Bridge C is therefore selected as the Designated Bridge Port
for LAN Segment 3.
Figure 11 shows three possible STP configurations using SuperStack 3
Switch units.
Configuration 1 — Redundancy for Backbone Link
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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
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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
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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.