You administer the network shown above. SwitchE is the root bridge for the network. You connect SwitchF to a port on SwitchB. SwitchF has a priority value of 0 and the MAC address 0000.0c42.0729.
Which statement is most accurate regarding root bridge selection after SwitchF is connected to SwitchB? (Select the best answer.)
- SwitchB will immediately become the root bridge.
- SwitchE will remain the root bridge.
- SwitchF will immediately become the root bridge.
- SwitchE will remain the root bridge until it is powered down, and then SwitchF will become the root bridge.
After you connect SwitchF to a port on SwitchB, SwitchF will become the root bridge because it has the lowest possible priority value and it has a lower Media Access Control (MAC) address than any of the other switches with a priority value of 0. The root bridge is the switch with the lowest bridge ID (BID), which is composed of a 2byte bridge priority and a 6byte MAC address. The bridge priority is considered first in the determination of the lowest BID. When two or more switches have the lowest priority, the switch with the lowest MAC address will become the root bridge. Because SwitchF has a lower MAC address than SwitchE, SwitchF will become the root bridge.
SwitchE will not remain the root bridge, because SwitchF has the same priority and a lower MAC address.
When a switch is powered on, it sends out bridge protocol data units (BPDUs) that contain the switch’s BID. As soon as a switch receives a BPDU with a lower BID than the current root switch BID, the switch will consider that BPDU to be superior, replace the root switch BID with the BID from the BPDU, and recalculate the root port and port costs. This can have an undesired effect on how packets are sent through a switched network. Therefore, when connecting a switch to a switched network, you must ensure that the switch has a higher priority value than the root bridge, unless you want the switch to assume the root bridge role. This is especially true if the switch is older or contains inferior technology, such as ports that are capable of only 10megabits per second (Mbps) transmission or halfduplex operation. Alternatively, you can issue the spanningtree guard root command to enable the root guard feature. The root guard feature, when enabled on a port, prevents superior BPDUs received on a neighbor switch connected to that port from becoming the root bridge. If superior BPDUs are received on a port enabled with root guard, the port enters the rootinconsistent state and the port is blocked until the port stops receiving superior BPDUs.
SwitchB will not become the root bridge. SwitchB has a priority value of 65535, which is the highest possible priority value. The root bridge is the switch with the lowest priority value. You can set the bridge priority by issuing the spanningtree priority value command, where value is a number from 0 through 65535? the default priority is 32768.
SwitchE will not remain the root bridge until it is powered down? SwitchF will immediately replace SwitchE as the root bridge. Root bridges do not behave the same as Open Shortest Path First (OSPF) designated routers (DRs) and backup DRs (BDRs) do. A DR is not replaced by another DR even if a router with a higher OSPF priority is introduced. A DR remains the DR until it fails or is powered down? then the BDR becomes the DR and a new BDR is selected.