19-9
Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
OL-13270-01
Chapter19 Configuring Optiona l Spa nning-Tree Features Understanding Optional Spanning-Tree Features
If link L1 fails as shown in Figure 19-7, Switch C cannot detect this failure because it is not connected
directly to link L1. However, because SwitchB is directly connected to the root switch over L1, it detects
the failure, elects itself the root, and begins sending BPDUs to SwitchC, 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 Swit ch C to move
immediately to the listening state without waiting for the maximum aging time for the interface to expi re.
BackboneFast then transitions the Layer 2 interface on SwitchC to the forwarding state, providing a path
from SwitchB to Switch A. The root-switch election takes approximately 30 seconds, twice the Forward
Delay time if the default Forward Delay time of 15 seconds is set. Figure19-7 shows how BackboneFast
reconfigures the topology to account for the failure of link L 1.
Figure19-7 BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure19-8, BackboneFast is
not activated because the inferior BPDUs did not come from the re cog niz ed designa te d sw itc h
(Switch B). The new switch begins sending inferior BPDUs that indicate it is the root switch. However ,
the other switches ignore these inferior BPDUs, and the new switch learns that Switch B is the
designated switch to SwitchA, the root switch.
Figure19-8 Adding a Switch in a Shared-Medium Topology
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Link failure
44964
BackboneFast changes port
through listening and learning
states to forwarding state.
Switch A
(Root)
S
witch C Switch B
(Designated bridg
e)
Added switch
4
4965
Blocked port