Proxy ARP confusion
Charles Scott (cscott@freeway.net)
Fri, 18 Apr 1997 00:09:59 -0400 (EDT)
I've tried a few times to configure some of our PM's to do proxy arp
for the assigned addresses and could never get it to work. Last night I
was trying again with a couple of our PM3's that we're putting in and ran
across something interesting.
I put both PM3's into the same class C (no subnets, mask .0) and tried
making a connection to one of them. The other PM3 would soon pick up
the addres of the connection to the other unit (sh arp ether0) but the
Cisco router that feeds both couldn't send packets to the user
connection.
I turned on debug on one of the portmasters and saw that the router
(an MGS running an old IOS version) was trying to find the MAC address
of the user connection, but there was a problem. The ethernet address
of the port on the router that is in the class C that the PM3's are in is
actually a secondary address for that port. The router was sending the
ARP requests from it's native port address, which isn't in that class C.
Now the questions (you knew I had some). I haven't yet found if there are
any commands to modify the router's behavior that way, but am looking. It is
however curious that the PM's debug output would show the arp requests for
one of it's addresses and not respond to it, even though the request
aparently came from an address outside of it's class C. Perhaps it really
can't respond to the request because of that. Does anyone have any thoughts
on this or do I need to go back and read how ARP really works? Anyone know
if ANY release of the Cisco IOS would make that arp request correctly? Also,
I'm wondering if this may be a problem others have had with proxy arp and
didn't know it.
Chuck Scott