Posted on January 10, 2022 11:04 am
 |  Asked by Ayo Oyekunle
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Hello Everyone

Am new to the DC network environment and am trying to figure out how to track down a connected host to TOR in a DC.  The steps I have taken:

i.  I ping the hostname and got the IP address

ii. I do tracert to the IP and find the last hop the host is connected to (layer 3)

iii. I connect to this device but when I run arp command I don’t get anything.

Am stuck at this point and I need your assistance because we don’t use software tool that should have done the mapping for us in our network.


HINT: Its a VXLAN design so am a bit confuse and still learning.

Posted by Aniket Bhowmick
Answered on January 10, 2022 11:14 am

Hi Ayo

Thanks for reaching out on EOS forum.

  1. After 3rd step, can you confirm if the L3 interface (SVI), where host is connected, part of any ip-vrf or not? You can run "show vrf" to see that interfaces shows up in any vrf or not. If it is part of a vrf, then run the command- "show arp vrf <vrf_name> <ip_address>". Example: show arp vrf red
  2. If you are unable to find the ARP on the switch, try doing a ping from that switch/VTEP. If L3 interface is part of a VRF, then do the ping using "ping vrf <vrf_name> <ip_address>". Ping may fail (which can happen in Vxlan setup, so you can ignore ping failure) but see if the ARP is learnt or not.
  3. If you do find the ARP learnt, try to locate the VTEP where the host is directly connected to. For that, run this command to track the mac address: "show vxlan address table | grep 001c.1234.4556" . <-- Here, instead of 001c.1234.4556, it should be the host mac. The output would give you a VTEP IP which is where the host is locally connected. Once the VTEP IP is found, go to that VTEP (where the VTEP IP exist) and then do a mac address lookup again using this- "show mac address-table | grep 001c.1234.4556" and that would give you the interface where the mac is locally learnt (that is where host is connected to).



Posted by Ayo Oyekunle
Answered on January 11, 2022 6:10 am

Thanks for providing the steps. For clarification, is there a command to identify the VTEP that represents the Loopback_IP address in the VXLAN or you have to use the design documentation.

Answered on January 11, 2022 3:24 pm

Hey Ayo,

If you want to check the VTEP's IP address, the command "show interface vxlan1" will show that.
If you are looking for the VTEP where a given MAC is, then you'd need to use "show vxlan address-table" which works similarly to the "show mac address-table" command but will list the VTEP from which a given MAC was learnt.


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