Branch switch and router arp question

wilson_1234_2 · ‎02-27-2008

We have remote branches and in each branch there are 2560 switches and 2811 routers.

Each branch is configured with 16 bit subnet masks.

Each device group is identified by the the octect of the IP address.

For example:

router = 10.10.1.1

switch = 10.10.2.1

server = 10.10.3.1

workstations = 10.10.4.x

The switches are configured with two VLANs, Voice and everything else. There are Cisco IP phones with the workstation plugged into the phone.

Everything is configured with the router as the default gateway in each branch.

My question is regarding the switch which only has the printers and non IP Phones in the arp table.

I can see everything including workstation and phones in the router arp table, which is trunking the two VLANs from the switch, but not the switch arp table.

What is the reason for this?

Richard Burts · ‎02-27-2008

Richard

This is a question that comes up from time to time. I find that the easiest way to answer it is to start by remembering some fundamentals: switches forward at layer 2 and their forwarding table remembers MAC addresses and not IP addresses. Routers forward at layer 3 and their forwarding table remembers IP addresses. But to build the packet to forward layer 3 IP addresses the router must also have the layer 2 MAC address for every locally connected device to which the router may forward.

So when you look at the ARP table of the router you will see all the devices locally connected to which it has forwarded packets (which is essentially every locally connected device).

A switch forwards at layer 2 and its mac-address-table will be complete. The switch management interface is a layer 3 interface. The switch acts as an IP host. So the switch maintains an ARP table for the various devices with which its management interface has communicated. So the ARP table on the switch is not all local devices but only the devices to which the switch management interface has communicated.

HTH

Rick

HTH

Rick

Joseph W. Doherty · ‎02-27-2008

Rick's post has the details about the ARP table, but as a side note, if you look at the switch's MAC table, you should see all the MACs that have passed through the switch.

wilson_1234_2 · ‎02-27-2008

you are correct, I have always seen a complete mac-address table in the remote switches.

Thanks.

wilson_1234_2 · ‎02-27-2008

Ahh yes,

why didn't I think of that.

My question came up because in the main site, the 6509's have the arp table in them, but they are layer three devices.

Thanks.