arp cache not showing the device connected at access switch

Unanswered Question
Mar 10th, 2008

when I normally need to trace a server connect to a switch, I trace from core > distribution > access.

1. Ping server

2. sh arp | inc ip_address

3. sh mac-add | inc 0000:0000:0000

Question is why do I normally get to the access switch, I do same squeence and I will not see it in the arp. But I use back the MAC I found intially and

sh mac-add | inc mac_add

I would find the port ? Why at access level the arp is not showing ?

Hopefully this is not to stupid question.

I have this problem too.
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Jon Marshall Mon, 03/10/2008 - 03:32

Hi

No, it's not a stupid question.

sh arp shows the L2 (mac-addresses) to L3 (IP address) mappings. So you would only find the server in the arp table on the L3 device which i'm guessing is your distribution device. By L3 device i mean the device that is responsible for the inter-vlan routing.

Jon

Richard Burts Mon, 03/10/2008 - 04:30

Loy

I am not clear when you say on the access switch you do the same sequence whether the same sequence includes the step of pinging or is only the show arp and show mac-address commands. Perhaps you can clarify?

If it is only the show commands then I believe that there is a simple explanation. Assuming that the access switch is a layer 2 switch, then it does maintain an ARP table (and the show arp command does work) but the content of the arp table contains only addresses which the management interface of the switch has accessed and not addresses for which it has done layer 2 forwarding. So if the ping was from the layer 3 switch or router then the mac-address table of the access switch would contain the mac address but the arp table would not (since the management interface of the switch did not access the device).

If the access switch does do the ping and the address still does not show up in the arp table then we may need to do some more digging into this issue.

HTH

Rick

loytakmem Mon, 03/10/2008 - 20:46

Its at the access switch which the server is directly connected which I perform the ping and sh arp to double confirmed. Ping works but the arp never show. But by utilizing the MAC I got on previous device I was able to find the actual switch port. Just wish to understand why this is such.

Thank you Rick for taking time to answer this.

Jon Marshall Mon, 03/10/2008 - 20:54

Hi

If you do a "sh arp" on the access switch what entries are there. A L2 switch will generally only have 1 entry which will be it's default-gateway so you will be able to ping the server from the access switch but it still will not show up in the arp table.

Jon

loytakmem Mon, 03/10/2008 - 21:48

oh ok, clearer picture now. Thank you Rick and Jon for the explanation on this question.

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