Cisco 2950G 24 Software Configuration Guide - Page 325

Chapter 13 Configuring Optional Spanning-Tree Features Understanding Optional Spanning-Tree Features If link L1 fails as shown in Figure 13-8, 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 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. Figure 13-8 shows how BackboneFast reconfigures the topology to account for the failure of link L1. Figure 13-8 BackboneFast Example After Indirect Link Failure Switch A (Root) Switch B L1 Link failure L2 L3 Switch C BackboneFast transitions port through listening and learning states to forwarding state. 44964 If a new switch is introduced into a shared-medium topology as shown in Figure 13-9, BackboneFast 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 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 13-9 Adding a Switch in a Shared-Medium Topology Switch A (Root) Switch C Blocked port Switch B (Designated bridge) Added switch 44965 78-14982-01 Catalyst 2950 Desktop Switch Software Configuration Guide 13-11