In an STP domain, which two statements are true for a nonroot switch, when it receives a
configuration BPDU from the root bridge with the TC bit set? (Choose two.)
A.
It sets the MAC table aging time to max_age + forward_delay time.
B.
It sets the MAC table aging time to forward_delay time.
C.
It recalculates the STP topology upon receiving topology change notification from the root switch.
D.
It receives the topology change BPDU on both forwarding and blocking ports.
When the TC bit is received, Every bridge is then notified and reduces the aging time to
forward_delay (15 seconds by default) for a certain period of time (max_age + forward_delay). It is
more beneficial to reduce the aging time instead of clearing the table because currently active
hosts, that effectively transmit traffic, are not cleared from the table.
Once the root is aware that there has been a topology change event in the network, it starts to
send out its configuration BPDUs with the topology change (TC) bit set. These BPDUs are relayed
by every bridge in the network with this bit set. As a result all bridges become aware of the
topology change situation and it can reduce its aging time to forward_delay. Bridges receive
topology change BPDUs on both forwarding and blocking ports.
An important point to consider here is that a TCN does not start a STP recalculation. This fear
comes from the fact that TCNs are often associated with unstable STP environments; TCNs are a
consequence of this, not a cause. The TCN only has an impact on the aging time. It does not
change the topology nor create a loop.
Reference: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-treeprotocol/
12013-17.html#topic1