Arp Table Entries For WAN Interfaces (NOT)

[ddarwactor]

I was just about to ask..."Why do two (of three) of my 2500 routers not have any Arp Table Entries?" when it dawned suddenly dawned on me as I was typing out the question here - one of the three is using the E0 interface connected to my LAN. The other two are only using S0 and S1.

I've been Googling this issue for a couple of days now trying to figure out why those two routers have absolutely no entries in the Arp Table even though they're configured correctly and I can ping every interface in my lab.

Then I got the email from Todd today about the Christams Exam Question gift (Thanks, again, Todd!!), which reminded me to check out your forum to see if anyone else had discussed a similar issue here. I didn't find anything so thought to ask the question. And subsequently realized I've been asking the Wrong Question. I should have been asking "Do serial interfaces contribute to the Arp Table?" When I googled that, the first hit at the top of the list was:
https://learningnetwork.cisco.com/thread/6498

I figured I'd post this here just in case some other poor befuddled soul has the great misfortune to wonder the same thing. It may already be mentioned in the book and I just haven't gotten to it yet...or maybe it was supposed to be intuitively obvious and my intuition was on the fritz!

BTW, still studying away...love the study guide, especially the Nook version that I carry around with me and can whip out easily during a work break!

[cableguy3443]

I have 3 routers and 1 and 2 WAN is connected via a serial but 2 and 3 is Fastethernet....can I not use this interface as a WAN? I am currectly using it as a WAN with a local network on a different ethernet port and routed that way but how does the ARP table fit in here and will there be any problems?

[ciscodaze]

no problems, you cant break arp :-)

PC, switches and routers all have arp tables, but we mostly talk about PCs and routers arping...because they do that - a lot. But switches have an arp table too, and people sometimes forget that, because it only has to do with remote management connections, like ssh, snmp, telnet, etc.

PC command line: arp -a
Router>sh arp
Switch>sh arp

Switches dont arp like a PC or a router, since they are really just transparent devices that forward, flood, or filter frames on the local lan, based on what they already know or dont know, then collect and log all the mac addresses that pass through them (talking L2 switches). We are mostly concerned with the mac address table here (also know as cam table).

Switch>sh mac


EDIT: sure you can use the FE as a wan - in PT :-)

Wan in the real world would be something other than ethernet, except for the exception of metro ethernet, which can go really long distances, 50 miles or whatever. So if distance is the definition of Wan, the metro ethernet could be called a wan.