×

Warning message

  • Cisco Support Forums is in Read Only mode while the site is being migrated.
  • Cisco Support Forums is in Read Only mode while the site is being migrated.

Determining the MAC of a remote switch?

Answered Question
Jun 20th, 2012
User Badges:

Remote as in not the one I have consoled into and not adjacent to that switch either (there are 4 "upstream" switches).  I have seen suggestions on older posts where it is recommended that we use sh cdp neigh detail to acquire the IP of the remote switch, ping it then check the local switches arp table via sh ip arp however I do not see how this would work.  CDP will only provide info on adjacent devices, correct?  I am trying to identify the root bridge (PVST) and I only have console access to one switch on the perimiter, no telnet ability.  Yes, this is for a test question which really threw me on my first attempt at the CCNA.  Any help/suggestions would be greatly appreciated!  Thanks!                 

Correct Answer by andrewswanson about 5 years 2 months ago

hello jeff


assuming this is for ccna - its been a while but i'm sure Cisco usually say "assume default values" in these exams. So SPT costs are 10Mbps (100), 100Mbps (19) and 1Gbps (4):


you have access to switch 1:


1. use show spanning-tree vlan X to find cost to Root bridge
2. if switch 1's root bridge cost is either 4,19, or 100 - then the root bridge is switch 2. if not, find the speed of the link between switch 1 and 2 and work out the SPT cost for this link.
3. Deduct the SPT cost worked out in step 2 from the original SPT cost (step 1) - this will give you switch 2's cost to the Root Bridge -  if switch 2's root bridge cost is either 4,19, or 100 - then the root bridge is switch 3. if not then the root bridge must be switch 4


maybe Cisco just wanted to check you knew the default values


hth
andy

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
andrewswanson Wed, 06/20/2012 - 06:10
User Badges:
  • Silver, 250 points or more

hello - on the switch you have access to, issue the command "show spanning-tree vlan X" ( where X is your vlan number). the output will show the root's mac address

hth

andy

jeffb_6538 Wed, 06/20/2012 - 06:16
User Badges:

Thanks Andy!  I was able to see the root's MAC however I could not determine which one of the "upstream" switches it belonged to.  That's the issue with this scenario.  I only have console to the last switch in the bus of switches.  I could determine if the root was the switch I'm consoled into or the adjacent to that but there are 3 other switches "upstream".  I hope I'm making sense. 

pnaduvat Wed, 06/20/2012 - 06:36
User Badges:
  • Cisco Employee,

Hi Jeff,


Just a curiosity, will "traceroute mac" be of any help here ?


Regards,

Prajul.


Message was edited by: Prajul Naduvath Sreedharan

andrewswanson Wed, 06/20/2012 - 06:40
User Badges:
  • Silver, 250 points or more

"sh spanning-tree vlan X" will also give you the SPT cost to the Root bridge. do you know the speed of the links connecting the switches?

hth

andy

jeffb_6538 Wed, 06/20/2012 - 06:46
User Badges:

I thought about determining the root via the cost's of the links, post-test and post-TRAUMA however I really don't think the link speeds were provided.  Essentially, this scenario/simulation was the reason I did not pass so I'm trying to ensure I can tackle it in case I see it again when I retake.   I've seen similar posts in previous years where the scenario is identical and like I said earlier, the suggestion was made to use CDP to acquire IP's of remote switches, ping them then check arp.  I don't see how that will help me since there is one switch next to my consoled switch and 3 more "upstream".  ???  

Correct Answer
andrewswanson Wed, 06/20/2012 - 07:20
User Badges:
  • Silver, 250 points or more

hello jeff


assuming this is for ccna - its been a while but i'm sure Cisco usually say "assume default values" in these exams. So SPT costs are 10Mbps (100), 100Mbps (19) and 1Gbps (4):


you have access to switch 1:


1. use show spanning-tree vlan X to find cost to Root bridge
2. if switch 1's root bridge cost is either 4,19, or 100 - then the root bridge is switch 2. if not, find the speed of the link between switch 1 and 2 and work out the SPT cost for this link.
3. Deduct the SPT cost worked out in step 2 from the original SPT cost (step 1) - this will give you switch 2's cost to the Root Bridge -  if switch 2's root bridge cost is either 4,19, or 100 - then the root bridge is switch 3. if not then the root bridge must be switch 4


maybe Cisco just wanted to check you knew the default values


hth
andy

jeffb_6538 Wed, 06/20/2012 - 08:00
User Badges:

Thanks Andy!  That's the way I will approach it if I see that simulator scenario again as I just don't see a way to obtain the MAC of the upstream switches given the limitations inherent in the problem.  Hopefully, I will not butcher that section and achieve a passing score on attempt number two!  Thanks again!

vijay.swaminathan Wed, 06/20/2012 - 08:14
User Badges:

I'm not sure if the command "show spanning tree vlan X detail" works here.. if so you can find out what is my cost and what is my neighbors cost to the root that it is advertising.


if the neighbors cost is 0 then we can assume that he is the root. if not, we can trace and move upward.


Just a thought.


-Vijay

jimmysands73_2 Wed, 06/20/2012 - 12:39
User Badges:
  • Silver, 250 points or more

Depending on the sim/allowed commands, you can also use


sh span root

Actions

This Discussion