Cisco Catalyst 2975 Software Configuration Manual page 484

Chapter 18 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
BackboneFast, which is enabled by using the spanning-tree backbonefast global configuration
command, starts when a root port or blocked interface on a switch receives inferior BPDUs from its
designated switch. An inferior BPDU identifies a switch that declares itself as both the root bridge and
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 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.
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.
Figure 18-6 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 18-6 BackboneFast Example Before Indirect Link Failure
Switch A
(Root)
Switch B
L1
L2 L3
Blocked port
Switch C
Catalyst 2975 Switch Software Configuration Guide
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