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Refer to the exhibit.
The network you administer consists of the devices shown in the exhibit. Each link is 100 megabits per second (Mbps) and is connected to a FastEthernet port. Switch S1 is the root bridge. You enable root guard on Fa0/0 on switch S2 and switch S3 by issuing the spanningtree guard root command in interface configuration mode on both switch ports. You also enable the UplinkFast feature on S2 and S3 by issuing the spanningtree uplinkfast command in global configuration mode on both switches.
Which of the following statements best describes what will occur if the link between S1 and S2 is broken? (Select the best answer.)
- Traffic will follow its normal path from Host2 to S1.
- The Fa0/0 port on both switches will be put into the rootinconsistent (blocked) state.
- Only Fa0/0 on S2 will be put into the rootinconsistent (blocked) state.
- Only Fa0/0 on S3 will be put into the rootinconsistent (blocked) state.
Explanation:
If the link between S1 and S2 is broken, the Fa0/0 port on S2 will be placed into the rootinconsistent state. When root guard is enabled on a port, it prevents a port from becoming a root port. Normally, a port that receives a superior bridge protocol data unit (BPDU) will become the root port. However, if a port configured with root guard receives a superior BPDU, the port transitions to the rootinconsistent state and the port will be blocked until it stops receiving superior BPDUs. As a result, root guard can be used to influence the placement of the root bridge on a network by preventing other switches from propagating superior BPDUs throughout the network and becoming the root bridge.
When the root bridge detects the broken link, it will send out BPDUs to converge the network topology. Since root guard was enabled on Fa0/0 on S2, the interface will be placed into the rootinconsistent state when it receives superior BPDUs from Fa0/0 on S3. Thus root guard prevents Fa0/0 on S2 from being selected as a root port. The port will remain in the rootinconsistent state until it stops receiving superior BPDUs from Fa0/0 on S3.
Fa0/0 on S3 will not be placed into the rootinconsistent state, because it will not receive superior BPDUs from S2. S3 will continue to receive superior BPDUs from S1.
Traffic would not follow its normal path from Host2 to the root bridge if the link between S1 and S2 were broken. When the link between S1 and S2 is up, traffic from Host2 travels from S4 to S2 to S1. This is based on the root path cost. The root path cost is an accumulation of path costs from bridge to bridge. A Fast Ethernet link has a path cost of 19. There are two 100megabits per second (Mbps) paths, so the root path cost from S4 to S2 to S1 equals 38. The root path cost from S4 to S3 to S1 also equals 38. If the root path cost is identical, the bridge ID (BID) is used to determine the path. In this scenario, S2 has a priority of 32768, as does S3. However, the Media Access Control (MAC) address for S2, 000000000002, is lower than the MAC address for S3, 000000000003, making S2 the designated bridge. If the link between S1 and S2 breaks, the path for traffic coming from Host2 will be rerouted from its normal path to the S4 to S3 to S1 path.
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