3Com 3824 Implementation Guide - Page 41

STP Configurations, Configuration 3 - Redundancy for Cabling Error

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How STP Works 41 ■ 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 12 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.

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How STP Works
41
■
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 12
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.