Cisco IE 3000, Руководство по настройке программного обеспечения

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20-6
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 20 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
The switch tries to find if it has an alternate path to the root switch. If the inferior BPDU arrives on a
blocked interface, the root port and other blocked interfaces on the switch become 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 interfaces become alternate paths to the root switch. If the
inferior BPDU arrives on the root port and there are no blocked interfaces, the switch assumes that it has
lost connectivity to the root switch, causes the maximum aging time on the root port to expire, and
becomes the root 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 RLQ request on all alternate paths and waits for an RLQ reply from
other switches in the network.
If the switch discovers that it still has an alternate path to the root, it expires the maximum aging time
on the interface 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
interface that received the RLQ reply. If one or more alternate paths can still connect to the root switch,
the switch makes all interfaces 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 learning
states, and into the forwarding state.
Figure 20-5 shows an example topology with no link failures. Switch A, the root switch, connects
directly to Switch B over link L1 and to Switch C over link L2. The Layer 2 interface on Switch C that
connects directly to Switch B is in the blocking state.
Figure 20-5 BackboneFast Example Before Indirect Link Failure
If link L1 fails as shown in Figure 20-6 , Switch C cannot detect this failure because it is not connected
directly to link L1. However, because Switch B is directly connected to the root switch over L1, it detects
the failure, elects itself the root, and begins sending BPDUs to Switch C, 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 interface on Switch C to move
immediately to the listening state without waiting for the maximum aging time for the interface to
expire. BackboneFast then transitions the Layer 2 interface on Switch C to the forwarding state,
providing a path from Switch B to Switch A. The root-switch election
takes approximately 30 seconds,
twice the Forward Delay time if the default Forward Delay time of 15 seconds is set. Figure 20-6 shows
how BackboneFast reconfigures the topology to account for the failure of link L1.
L1
L2 L 3
S witch C
S witch A
(Root)
S witch B
Blocked port
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3