Cisco 2950 - Catalyst Switch Configuration Manual page 352

Understanding Optional Spanning-Tree Features
If link L1 fails as shown in
directly to link L1. However, because Switch B is directly connected to the root switch over L1, it detects
the failure, elects itself the root, and begins sending BPDUs to Switch C, 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 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 Layer 2 interface on Switch C to the forwarding state, providing a path from Switch B to
Switch A. This switchover takes approximately 30 seconds, twice the Forward Delay time if the default
Forward Delay time of 15 seconds is set.
topology to account for the failure of link L1.
Figure 16-8 BackboneFast Example After Indirect Link Failure
Switch A
(Root)
If a new switch is introduced into a shared-medium topology as shown in
not activated because the inferior BPDUs did not come from the recognized designated bridge
(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 bridge to Switch A, the root switch.
Figure 16-9 Adding a Switch in a Shared-Medium Topology
Switch C
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
16-10
Figure 16-8, Switch C cannot detect this failure because it is not connected
L1
Link failure
L2
BackboneFast changes port
through listening and learning
states to forwarding state.
Switch C
Switch A
(Root)
Blocked port
Chapter 16
Figure 16-8 shows how BackboneFast reconfigures the
Switch B
L3
Switch B
(Designated bridge)
Added switch
Configuring Optional Spanning-Tree Features
Figure 16-9, BackboneFast is
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