Cisco 3560G 48TS Software Configuration Guide - Page 353

Chapter 17 Configuring Optional Spanning-Tree Features Understanding Optional Spanning-Tree Features Figure 17-6 BackboneFast Example After Indirect Link Failure Switch A (Root) Switch B L1 Link failure L2 L3 Switch C BackboneFast changes 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 17-7, BackboneFast is not activated because the inferior BPDUs did not come from the recognized designated switch (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 Switch A, the root switch. Figure 17-7 Adding a Switch in a Shared-Medium Topology Switch A (Root) Switch C Blocked port Switch B (Designated bridge) Added switch 44965 Understanding Root Guard The Layer 2 network of a service provider (SP) can include many connections to switches that are not owned by the SP. In such a topology, the spanning tree can reconfigure itself and select a customer switch as the root switch, as shown in Figure 17-8. You can avoid this situation by enabling root guard on SP switch interfaces that connect to switches in your customer's network. If spanning-tree calculations cause an interface in the customer network to be selected as the root port, root guard then places the interface in the root-inconsistent (blocked) state to prevent the customer's switch from becoming the root switch or being in the path to the root. 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 17-7