Consolidated Platform Configuration Guide, Cisco Ios Release 15.2(4)E (Catalyst 2960-X Switches - Cisco Catalyst 2960 series Configuration Manual

Information About Optional Spanning-Tree Features
switch). Under spanning-tree rules, the switch ignores inferior BPDUs for the maximum aging time (default
is 20 seconds).
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 to learn if any stack member has an alternate
root to the root switch and waits for an RLQ reply from other switches in the network and in the stack. The
switch sends the RLQ request on all alternate paths and waits for an RLQ reply from other switches in the
network.
When a stack member receives an RLQ reply from a nonstack member on a blocked interface and the reply
is destined for another nonstacked switch, it forwards the reply packet, regardless of the spanning-tree interface
state.
When a stack member receives an RLQ reply from a nonstack member and the response is destined for the
stack, the stack member forwards the reply so that all the other stack members receive it.
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.
This is 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 25: BackboneFast Example Before Indirect Link Failure
If link L1 fails, 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

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