Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 279

Chapter 18 Configuring Optional STP Features Understanding How BackboneFast Works Figure 18-3 Switch A (Root) BackboneFast Example Before Indirect Link Failure Switch B L1 L2 L3 Blocked port Switch C 11241 If link L1 fails, Switch C cannot detect this failure because it is not connected directly to link L1. However, because Switch B is directly connected to the root bridge over L1, it detects the failure and elects itself the root and begins sending BPDUs to Switch C indicating itself as the root. When Switch C receives the inferior BPDUs from Switch B, Switch C infers 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 LAN 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. Figure 18-4 shows how BackboneFast reconfigures the topology to account for the failure of link L1. Figure 18-4 Switch A (Root) BackboneFast Example After Indirect Link Failure L1 Link failure Switch B L2 L3 Switch C BackboneFast transitions port through listening and learning states to forwarding state 11244 If a new network device is introduced into a shared-medium topology as shown in Figure 18-5, BackboneFast is not activated because the inferior BPDUs did not come from the recognized designated bridge (Switch B). The new network device begins sending inferior BPDUs that indicate that it is the root bridge. However, the other network devices ignore these inferior BPDUs and the new network device learns that Switch B is the designated bridge to Switch A, the root bridge. OL-11439-03 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY 18-5