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).
“Avoiding HSRP Instability in a Switching Environment with Various Router
Platforms”http://www.cisco.com/en/US/tech/tk648/tk362/technologies_configuration_example09186a00
80093f93.shtml
B is obviously wrong as stated above. STP boundaries are layer2, so stop at the layer 3 routers.
answer A is deceiving… it lures you… think about it though… its telling you spanning tree wont converge because rtb is taking over a router…. that’s not correct. spanning tree is not coverging because nothing has changed (nothing to do with rtb).. all the interfaces that were previously up are still up (in its broadcast domain).
Answer C must therefore be correct. RTB becomes hsrp master and starts receiving gateway traffic.
thanks all.
tony.
None of the answers are correct. The failure of the serial link has no effect on spanning tree. You won’t run hsrp on a point to point link. The layer 2 path between b and c is unaffected therefore router c remains the active router.
The only way you could get the topology to react to the failure of an (unused) interface is by configuring interface state tracking to affect the priority. That would be completely dumb has why would you track a layer 3 interface when you are directly connected to the network you are providing hsrp for ???