Cisco WS-C3550-12G Software Configuration Manual page 297

Chapter 10 Configuring STP
switchover takes approximately 30 seconds, twice the Forward Delay time if the default Forward Delay
time of 15 seconds is set.
for the failure of link L1.
Figure 10-11 BackboneFast Example After Indirect Link Failure
Switch A
(Root)
If a new switch is introduced into a shared-medium topology as shown in
is not activated because the inferior BPDUs did not come from the recognized designated bridge
(Switch B). The new switch begins sending inferior BPDUs that say it is the root switch. However, the
other switches ignore these inferior BPDUs, and the new switch learns that Switch B is the designated
bridge to Switch A, the root switch. For more information, see the
on page
Figure 10-12 Adding a Switch in a Shared-Medium Topology
Switch C
78-11194-03
Figure 10-11
L1
Link failure
L2
Switch C
10-36.
Switch A
(Root)
Blocked port
shows how BackboneFast reconfigures the topology to account
Switch B
L3
BackboneFast transitions port
through listening and learning
states to forwarding state.
Switch B
(Designated bridge)
Added switch
Catalyst 3550 Multilayer Switch Software Configuration Guide
Understanding Advanced STP Features
Figure 10-12, BackboneFast
"Configuring BackboneFast" section
10-19