Understanding How Backbonefast Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Chapter 9 Configuring Spanning Tree PortFast, UplinkFast, BackboneFast, and Loop Guard

Understanding How BackboneFast Works

Understanding How BackboneFast Works

BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs from its
designated bridge. An inferior BPDU identifies one switch as both the root bridge and the designated
bridge. When a switch receives an inferior BPDU, it indicates 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 bridge). Under normal spanning tree rules, the switch ignores inferior BPDUs for the configured
maximum aging time, as specified by the agingtime variable of the set spantree maxage command.
The switch tries to determine if it has an alternate path to the root bridge. 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
bridge. (Self-looped ports are not considered alternate paths to the root bridge.) If the inferior BPDU
arrives on the root port, all blocked ports become alternate paths to the root bridge. 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 bridge, causes the maximum aging time on the root to expire, and becomes the root switch
according to normal spanning tree rules.
If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new kind of
PDU called the Root Link Query PDU out all alternate paths to the root bridge. If the switch determines
that it still has an alternate path to the root, it causes the maximum aging time on the ports on which it
received the inferior BPDU to expire. If all the alternate paths to the root bridge indicate that the switch
has lost connectivity to the root bridge, the switch causes the maximum aging times on the ports on
which it received an inferior BPDU to expire. If one or more alternate paths can still connect to the root
bridge, the switch makes all ports on which it received an inferior BPDU its designated ports and moves
them out of the blocking state (if they were in the blocking state), through the listening and learning
states, and into the forwarding state.
Figure 9-3 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 port on Switch C that connects directly to
Switch B is in the blocking state.
Figure 9-3 BackboneFast Example Before Indirect Link Failure
Switch A
Switch B
(Root)
L1
L2 L3
Blocked port
Switch C
If link L1 fails, Switch C detects this failure as an indirect failure, since it is not connected directly to
link L1. Switch B no longer has a path to the root switch. BackboneFast allows the blocked port on
Switch C to move immediately to the listening state without waiting for the maximum aging time for the
port to expire. BackboneFast then transitions the port on Switch C to the forwarding state, providing a
path from Switch B to Switch A. This switchover takes approximately 30 seconds. Figure 9-4 shows
how BackboneFast reconfigures the topology to account for the failure of link L1.
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
9-4
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