What is the role of ARP on a switch?

surama123 · ‎01-30-2009

I have understood that layer 2 switch is only interested in MAC addresses so why do switches keep an ARP table?

We have some issues with a client who has connected a switch between multiple WAN connections and an ISA server. Each of the WAN devices is on a separate subnet, some are offering DHCP. My common sense tells me that this is not a good setup as there is one broadcast domain and multiple subnets attached to it. I would appreciate any pointers to understand better why this is not working well.

paolo bevilacqua · ‎01-30-2009

Hi, you said the reason already: multiple subnets, multiple DHCP, one physical media = bad.

These are usually the kind of mistakes made by end-users that think they know better because have been lucky in the past.

Joseph W. Doherty · ‎01-30-2009

"I have understood that layer 2 switch is only interested in MAC addresses so why do switches keep an ARP table? "

Because managed L2 switches are also IP hosts, and like other IP hosts, they maintain an ARP table.

Agree with Paolo that multiple subnets within a single broadcast domain is likely "bad".