consecutive times, protocol information can be immediately aged out (or if max_age expires). Because of the previously mentioned protocol modification, BPDUs are now used as a keep-alive mechanism between bridges. A bridge considers that it loses connectivity to its direct neighbor root or designated bridge if it misses three BPDUs in a row. This fast aging of the information allows quick failure detection. If a bridge fails to receive BPDUs from a neighbor, it is certain that the connection to that neighbor is lost. This is opposed to 802.1D where the problem might have been anywhere on the path to the root. You are the administrator of a switch and currently all host-connected ports are configured with the portfast command. You have received a new directive from your manager that states that, in the future,
any host-connected port that receives a BPDU should automatically disable PortFast and begin transmitting BPDUs. Which of the following commands will support this new requirement? A. Switch(config)# spanning-tree portfast bpduguard default B. Switch(config-if)# spanning-tree bpduguard enable C. Switch(config-if)# spanning-tree bpdufilter enable D. Switch(config)# spanning-tree portfast bpdufilter default Answer: D Explanation The bpdufilter option feature is used to globally enable BPDU filtering on all Port Fast-enabled interfaces and this prevent the switch interfaces connected to end stations from sending or receiving BPDUs. Note: The spanning-tree portfast bpdufilter default global configuration command can be overridden by the spanning-tree bdpufilter enable command in interface mode. Which two statements correctly describe characteristics of the PortFast feature? (Choose two) A. STP will be disabled on the port. B. PortFast can also be configured on trunk ports. C. PortFast is required to enable port-based BPDU guard. D. PortFast is used for both STP and RSTP host ports.
E. PortFast is used for STP-only host ports. Answer: B D Explanation You can use PortFast on switch or trunk ports connected to a single workstation, switch, or server to allow those devices to connect to the network immediately, instead of waiting for the port to transition from the listening and learning states to the forwarding state -> B is correct. Also, PortFast can be used for both STP and RSTP -> D is correct. Why is BPDU guard an effective way to prevent an unauthorized rogue switch from altering the spanning-tree topology of a network? A. BPDU guard can guarantee proper selection of the root bridge. B. BPDU guard can be utilized along with PortFast to shut down ports when a switch is connected to the port. C. BPDU guard can be utilized to prevent the switch from transmitteing BPDUs and incorrectly altering the root bridge election. D. BPDU guard can be used to prevent invalid BPDUs from propagating throughout the network.
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- Fall '17
- root bridge, BPDUs