Redundancy ( or lack of) on the TSHOOT topology

johnwest43johnwest43 Member Posts: 294
I found an interesting scenario in the TSHOOT topology and wanted to see if anyone wanted to comment on it. I have recreated the topo using only the information given by Cisco using real hardware. I have also attached the topology from Cisco's website.

At first glance the L2 topology looks redundant. However while testing some DHCP snooping features I shutdown fa0/19 and fa0/20 on ASW1. Here is where the fun begins.

At the point that these interfaces are shut down both DSW1 and DSW2 think they are the active HSRP router. I begin to wonder why this happened and I begin to trouble shoot. I find that Client 1 can no longer ping to or past R4. I do a trace route from R4 to Client 1 and it verifies that R4 is load balancing between DSW1 and DSW2 which is to be expected as they both have the same metric to reach Client 1.

At this point I remember that the ether channel between DSW1 and DSW2 is an L3 ether channel. This tells me that the only way HSRP can function for VLAN 10 is through ASW1.
Now I can’t say for sure what other commands may or may not be configured in the exam but this would certainly through you for a loop if you were in a time crunch!

Thoughts? ……..
Comments? ……..
Have I gone mad? ........ allout.gif
CCNP: ROUTE B][COLOR=#ff0000]x[/COLOR][/B , SWITCH B][COLOR=#ff0000]x[/COLOR][/B, TSHOOT [X ] Completed on 2/18/2014


  • networker050184networker050184 Mod Posts: 11,962 Mod
    I haven't looked at it, but if it is a L3 link between the two then L2 through the access switches would be the only path for the keepalives.

    It is a topology built for troubleshooting issues so it wouldn't make much sense to have an unbreakable topology!
    An expert is a man who has made all the mistakes which can be made.
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