Understanding Backbonefast - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 18 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features

Understanding BackboneFast

BackboneFast detects indirect failures in the core of the backbone. BackboneFast is a complementary
technology to the UplinkFast feature, which responds to failures on links directly connected to access
switches. BackboneFast optimizes the maximum-age timer, which determines the amount of time the
switch stores protocol information received on an interface. When a switch receives an inferior BPDU
from the designated port of another switch, the BPDU is a signal that the other switch might have lost
its path to the root, and BackboneFast tries to find an alternate path to the root. The BackboneFast feature
is supported only when the switch is running PVST+. It is not supported when the switch is running rapid
PVST+ or MSTP.
BackboneFast, which is enabled by using the spanning-tree backbonefast global configuration
command, starts when a root port or blocked port 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 bridge 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 port, the root port and other blocked ports 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 ports become alternate paths to the root switch. If the inferior 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 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 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 can still connect to the root switch, the switch
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 learning states, and into the
forwarding state.
Figure 18-7 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.
Catalyst 3550 Multilayer Switch Software Configuration Guide
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