Scenario 4 Answers

1. The spanning-tree topology should look like the diagram in Figure 20-9. Catalyst A is the root bridge, and only the 1000-Mbps link is forwarding. The root ports (RP) and designated ports (DP) are labeled on the diagram.

Figure 20-9 Resulting Spanning-Tree Topology for Scenario 4

2. Because the 100-Mbps link is in the Blocking state on Catalyst B, no major change in the topology occurs. Effectively, this link already was "disconnected." However, after the physical link status goes down, both Catalyst A and Catalyst B sense the change and begin sending TCN BPDUs to notify each other of the topology change. Because Catalyst A is the root bridge, it acknowledges the TCN to Catalyst B. Both switches age out their MAC address tables in Forward Delay seconds.

3. Disconnecting the 1000-Mbps link causes Catalyst B to immediately find another root port. Ports 1/1 and 1/2 go into the Listening state, waiting to receive BPDUs. Port 1/2, with a cost of 19, becomes the next root port as soon as Catalyst B computes the root path cost (0 +19) for it. Port 1/2 stays in the Listening state for Forward Delay (15 seconds), and then in the Learning state for Forward Delay (15 seconds). Port 1/2 moves into the Forwarding state, restoring connectivity in 30 seconds. (If PAgP is operating on the port, an additional delay of 20 seconds occurs.)

4. Because the 1000-Mbps link's status stays up, neither Catalyst detects a link failure. Therefore, no immediate attempt to find another root port occurs. Instead, Catalyst B will not receive BPDUs from Catalyst A over link GigabitEthernet 2/1 because they are being filtered out. After the MaxAge Timer expires (20 seconds), Catalyst B ages out the stored BPDU for Catalyst A on port GigabitEthernet 2/1. Catalyst B moves ports FastEthernet 1/1 and 1/2 into the Listening state to determine a new root port. As in step 3, port FastEthernet 1/2 becomes the root port with a lower root path cost than port FastEthernet 1/1. The port moves through the Listening (15 seconds) and Learning (15 seconds) states and into the Forwarding state. The total time that has elapsed before connectivity restores is 20 + 15 + 15 = 50 seconds. (Again, if PAgP is active on the port, an additional 20 seconds can be added to the delay.)

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