Observe the topology in the exhibit. HSRP is configured between RTB and RTC with RTC as the
active router. SW2 is configured as the root bridge for the Spanning Tree Protocol. What will
happen if the serial connection on RTC is down?

A.
STP will not need to be recalculated because RTB will take over as active router.

B.
RTB and RTC will flap between active and standby because the timers for STP are greater than
the timers for HSRP.

C.
All traffic will automatically forward to RTB.

D.
SW3 will take over as the new root bridge.

Explanation:
When you run the Hot Standby Router Protocol (HSRP) between two routers connected via a LAN
switch, you may observe instability in HSRP. This often happens during a network disruption or an
active router transition; such as an HSRP router with a higher priority and preemption configured
being added to the LAN. In the reference link this problem is described and one of the solutions is
to change the HSRP timers so that the spanning tree forward delay (default of 15 seconds) is less
than half the HSRP holdtime (default of 10 seconds).
Reference: “Avoiding HSRP Instability in a Switching Environment with Various Router Platforms”
http://www.cisco.com/en/US/tech/tk648/tk362/technologies_configuration_example09186a008009
3f93.shtml