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Catalyst 2940 Switch Software Configuration Guide
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Chapter12 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
The switch tries to determine if it has an alternate path to the root switch. If the inferior BPDU arrives
on a blocked port, the root port and other blocked ports on the switch bec ome alternate paths to the root
switch. (Self-looped ports are not considered alternate paths to the root switch.) If the inferior BPDU
arrives on the root port, all blocked ports become alternate paths to th e root switch . If the inferi or BPDU
arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity
to the root switch, causes the maximum aging time on t he ro ot po rt to expi re, and be co mes the ro ot
switch according to normal spanning-tree rules.
If the switch has alternate paths to the root switch, it uses these alternate paths to send a root link query
(RLQ) request. The switch sends the RLS request on all alternate paths to the root switch and waits for
an RLQ reply from other switches in the network.
If the switch determines that it still has an alternate path to the root, it expires the maximum aging time
on the port that received the inferior BPDU. If all the alternate paths to the root switch indicate that the
switch has lost connectivity to the root switch, the switch expires the maximum aging time on the port
that received the RLQ reply. If one or more alternate paths ca n still connec t to the root switch, the swi tch
makes all ports on which it received an inferior BPDU its designated ports and moves them from the
blocking state (if they were in the blocking state), through the listening and lea rning states, and into the
forwarding state.
Figure 12-5 shows an example topology with no link failures. Switch A, the root switch, connects
directly to SwitchB over link L1 and to Switch C over link L2. The Layer 2 interface on SwitchC that
connects directly to Switch B is in the blocking state.
Figure12-5 BackboneFast Example Before Indirect Link Failure
If link L1 fails as shown in Figure 12-6, Switch C cannot detect this failure because it is not connected
directly to link L1. However, because SwitchB is directly connected to the root switch over L1, it detects
the failure, elects itself the root, and begins sending BPDUs to SwitchC, identifying itself as the root.
When Switch C receives the inferior BPDUs from Switch B, Switch C assumes that an indirect failure
has occurred. At that point, BackboneFast allows the blocked port on Switch C to move immediately to
the listening state without waiting for the maximum aging time for the port to expire. Bac kboneFast then
transitions the Layer 2 interface on SwitchC to the forwarding state, providing a path from Switch B to
Switch A. This switchover takes approximately 30 seconds, twice the Forward Delay time if the default
Forward Delay time of 15 seconds is set. Figure12-6 shows how BackboneFast reconfigures t he
topology to account for the failure of link L1.
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Blocked port
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