sh ip arp does not show output.

[mahesh18]

                   Hi Everyone,

Server is connected to  say switch A on  vlan y.

vlan Y has SVI inetrface on another switch B.

sh mac add     address table works fine and shows server mac address as it is directly connected to switch A.I can ping server from switch A.

But when i do sh ip arp server IP there is no output.

Need to know output is blank as SVI is on another switch?

Thanks

Mahesh

[Reza Sharifi · Accepted solution]

Hi Mahesh,

If the SVI interface is on switch B, that is where you will see the IP address.

On switch B, "sh ip arp vlan x" should show you the IP address of the server.

Switch A is just layer-2 and as you already know it will give you the MAC address of the server not the IP.

HTH

Reza

[mahesh18]

Hi Reza,

Switch A is layer 3 switch do you mean it say it is layer 2 for that vlan?

thanks

Mahesh

[Richard Burts · Accepted solution]

Mahesh

When you tell us that the server is connected to switch A and that the SVI for that vlan is on switch B then that is an indication that switch A is acting only at layer 2 for that vlan. And that is the reason why the MAC of the server does not show up in the results of show arp on switch A. As I atempted  to explain the arp table will have entries only for locally connected devices to which the switch has done layer 3 forwarding.

HTH

Rick

[mahesh18]

Hi Rick,

Its always pleasure to read answers from you.

Regards

Mahesh

[Richard Burts · Accepted solution]

Mahesh

When you are on a switch and do show arp it will show results that are devices that are connected in the same subnet/same vlan as the management interface of the switch. It will not show the MAC of devices that are connected to the switch but are in vlan different from the vlan of the management interface of the switch.

When you think about it arp is used to resolve the MAC address for a device that you are communicating with at layer 3. When the switch does ping to the server and the server is not in the vlan/subnet of the management interface then the switch forwards the request to its default gateway (on the other switch) and the device with the layer 3 interface in the subnet/vlan of the server is the device that will have the MAC of the server in its arp table.

HTH

Rick

[mahesh18]

Hi Rick,

Seems what you said is absolutely correct.

As switch A  has many SVI.

when i do sh arp  on it it show me all the IP and mac address of the vlans which have SVI on switch A.

As Vlan Y SVI is on another switch thats reason sh arp does  not show mac and ip of vlan Y.

Sometimes it is good to know the reasons for small things like sh ip arp  in network world.

Best regards

Mahesh

[Richard Burts · Accepted solution]

Mahesh

I agree that it can be very good to understand some of the small things that happen in the network like the output of show arp.

I find that understanding how arp works can be very helpful and useful in troubleshooting certain types of network problems. For example I was recently working on an issue where a host was not able to communicate with its configured gateway router. By using the results of arp we were able to demonstrate that there was correct layer 2 communication and that led us to find that there was a layer 3 configuration error that was preventing communication.

HTH

Rick

[Leo Laohoo · Accepted solution]

Hi Mahesh,

Are you trying to determine what is the IP address of a server connected to a particular port?  There's another way that my colleague have found.  Try this method:

conf t

ip device tracking

interface

ip device tracking maximum 10

end

--- Wait for 15 seconds ---

Command:  sh ip device tracking all

This will show you the IP address even though the IP address is on a different VLAN.

[mahesh18]

Hi Leolaohoo,

I already know the IP of server connected to switchport.

But thanks for letting us very usefull  command.

This Forum is best place to learn.

Regards

MAhesh

[Leo Laohoo]

Thanks for taking the time to rate our posts, Mahesh.

[andrewgbrown]

This is a great tip, thank you.