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”

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