Cisco WS-C3560-48PS-S Software Configuration Manual page 352

Chapter 17 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
the designated switch. When a switch receives an inferior BPDU, it means that a link to which the switch
is not directly connected (an indirect link) has failed (that is, the designated switch has lost its connection
to the root switch). Under spanning-tree rules, the switch ignores inferior BPDUs for the configured
maximum aging time specified by the spanning-tree vlan vlan-id max-age global configuration
command.
The switch tries to determine 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 17-5 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 17-5 BackboneFast Example Before Indirect Link Failure
Switch A
(Root)
Switch B
L1
L2 L3
Blocked port
Switch C
If link L1 fails as shown in Figure 17-6, 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. This switchover takes approximately 30 seconds, twice the Forward Delay
time if the default Forward Delay time of 15 seconds is set. Figure 17-6 shows how BackboneFast
reconfigures the topology to account for the failure of link L1.
Catalyst 3560 Switch Software Configuration Guide
17-6
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