Solved: ARP table on Cisco 3550

ksarin123_2 · ‎03-03-2009

Hello folks -

I have a couple of Cisco 3550 48 ports switch running 12.1 IOS version. When I issue the command "show arp" on either of the switches, I get a very short arp table which only lists switches that the switch in question is connected to. I do not see any host IP's listed in the ARP table. The switches have atleast 30 hosts individually connected to each of them.

So why would the ARP table be so short?

Thanks for your help!!

Giuseppe Larosa · ‎03-03-2009

Hello Kunal,

you can search by ip address but you need to look at the right device:

you go on the default gateway

sh ip arp x.y.z.w

note the mac address

go on the switch and look for

sh mac-address-table address

so you can find the port where a PC with ip x.y.z.w is connected

Hope to help

Giuseppe

View solution in original post

Giuseppe Larosa · ‎03-03-2009

hello Kunal,

a L2 switch keeps the learned MAC addresses in the CAM table.

to see hosts MAC addresses use:

sh mac-address-table

(there are possible variants of the command syntax older IOS version can be

sh mac-address table)

Conversely the ARP table contains only the entries of devices that have a L3 (IP ) communication with the TCP/IP stack of the switch processor.

For this reason it looks like short in comparison to the number of connected hosts.

if you have a separate management Vlan the switch will learn the MAC address of its default gateway and of other network devices in the same subnet.

Hope to help

Giuseppe

ksarin123_2 · ‎03-03-2009

Giuseppe -

Thanks for your prompt reply.

So if I understand you correctly, there is no way for me to find out what port a certain workstation is connected to without knowing the machine's mac address. Correct?

Also, then why does my Cisco 4500 switch show the IP address of all the host workstations that are connected to it when I look at the arp table on the 4500?

Thanks again!

Giuseppe Larosa · ‎03-03-2009

Hello Kunal,

you can search by ip address but you need to look at the right device:

you go on the default gateway

sh ip arp x.y.z.w

note the mac address

go on the switch and look for

sh mac-address-table address

so you can find the port where a PC with ip x.y.z.w is connected

Hope to help

Giuseppe