No, Priscilla, you are not being brain dead and yes, Priscilla, there are
those of us who appreciate your perspective on this and other topics.
It turns out that there is an RFC 1027 which discusses proxy arp /
promiscuous arp / arp hack ( all terms Comer uses ) it would appear from my
quick skim, that originally proxy arp dealt with physically connected
subnets, but that a router performing proxy arp services can indeed use as
its knowledge base the entire routing table.
As to the situation in the hotel room I would suspect that if this were an
ethernet LAN, and my laptop had a fixed IP config for it's ethernet port,
that there would be a stream of issues. Dial up, where the dial up adapter
usually does not have a fixed address, would be a different situation.
BTW, Bob Vance would enjoy reading this RFC, I imagine. It gets into the
question about how broadcasts are handled, and seems to deal with a Bob
question I recall from long ago - how do hosts on different subnets but on
the same physical wire deal with arp requests.
IE the ARP broadcast goes out to the all F's . why is routing required in
such a situation. All hosts would see the ARP requests, even if on different
subnets, because al hosts are on the same wire.
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Priscilla Oppenheimer
Sent: Friday, May 25, 2001 12:04 PM
To: [EMAIL PROTECTED]
Subject: RE: ARP versus Proxy-arp [7:5664]
If a router running Proxy ARP didn't have a "route of last resort" or
"default route" would it still respond to an ARP for some random non-local
network? It would cause problems if it responded to the ARP when it
couldn't really route packets to the destination. I suppose it usually
works because this router or the DG as you mention below has a default
route to the rest of the world.
And how about network 10.0.0.0? The hotel router in the scenario wouldn't
respond to a customer's ARP for a DG of 10.0.0.1 unless the hotel network
was configured with a 10.0.0.0 network, would it? Or maybe the default
route would cover this too, but maybe not since it's a private address.
I realize I'm being brain damaged about the whole topic, but I think the
issues are more subtle than people realize.
Priscilla
At 09:14 PM 5/24/01, Bob Vance wrote:
> >Why would it think it can get to 10.0.0.0 (that ones a little
> >easier) or 138.1.0.0 (unlikely) when the client computer ARPs for its
> >default gateway?
>
>Well, now.
>Does a DG of its own count as "knowing how to get there"?>)
>
>
>-------------------------------------------------
>Tks |
>BV |
>Sr. Technical Consultant, SBM, A Gates/Arrow Co.
>Vox 770-623-3430 11455 Lakefield Dr.
>Fax 770-623-3429 Duluth, GA 30097-1511
>=================================================
>
>
>
>
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
>Priscilla Oppenheimer
>Sent: Thursday, May 24, 2001 6:24 PM
>To: [EMAIL PROTECTED]
>Subject: RE: ARP versus Proxy-arp [7:5664]
>
>
>You missed the point. I know what Proxy ARP is.
>
>I assume the goal is that the traveller doesn't need to do any
>reconfiguration and can leave the default gateway set to the home office
>setting of 10.0.0.32, or 138.1.80.193 in my second example. A router
>doesn't just blindly respond to ARPs. It only responds if it thinks it
>can
>get there. Why would it think it can get to 10.0.0.0 (that ones a little
>easier) or 138.1.0.0 (unlikely) when the client computer ARPs for its
>default gateway?
>
>The design of the hotel network must be quite interesting. I was hoping
>the
>original poster had more details.
>
>Priscilla
>
>At 12:35 PM 5/24/01, Cornell Manea wrote:
> >Proxy-arp is used to find a router and get by on a
> >segment when you don't know the IP address of the
> >default gateway...
> >
> >
> >--- Priscilla Oppenheimer wrote:
> > > Hmm... That's interesting. I'm trying to figure it
> > > out. Say, on my office
> > > network, my default gateway is something like
> > > 10.0.0.32 because we're using
> > > private addresses and NAT. When I travel, would the
> > > router in the hotel
> > > respond to my ARP for 10.0.0.32?? Would the router
> > > think that it can reach
> > > network 10.0.0.0?
> > >
> > > And, let's say that I don't use private addresses on
> > > my office network
> > > (which I don't). Let's say the default gateway is
> > > 138.1.80.193. Would the
> > > hotel router respond to my ARP for 138.1.80.193?
> > > Would the router think
> > > that it can reach network 138.1.0.0?
> > >
> > > I would hate to be the desk clerk responding to
> > > questions about this! ;-)
> > >
> > > Priscilla
> > >
> > > At 10:56 AM 5/24/01, [EMAIL PROTECTED] wrote:
> > > >Proxy-Arp Lives!
> > > >
> > > >I have to add that as I understand it proxy arp and
> > > nat are how hotels offer
> > > >internet connectivity. Take a laptop with any ip
> > > address configured plug it
> > > >in and it will arp for its default gateway. The
> > > router with proxy arp will
> > > >answer as the default gateways mac address. Then
> > > using a wide scope for nat
> > > >(the scope would be the entire ip address range)
> > > the hotel can provide
> > > >internet connectivity to a client with any
> > > configured ip address and
> > > >gateway.
> > > >
> > > >Dean Whitley
> > > >
> > > >-----Original Message-----
> > > >From: Hire, Ejay [mailto:[EMAIL PROTECTED]]
> > > >Sent: Thursday, May 24, 2001 10:32 AM
> > > >To: [EMAIL PROTECTED]
> > > >Subject: RE: ARP versus Proxy-arp [7:5664]
> > > >
> > > >
> > > >Proxy arp isn't dead, it is still in use very
> > > frequently on dial-up links.
> > > >If you get a chance, dial-up to earthlink and run
> > > winipcfg. You'll see that
> > > >your default gateway is actually set to yourself.
> > > Their is a reasonable
> > > >explanation of this behavior in the Sybex CCNP
> > > switch 2.0 chapter on
> > > >redundancy.
> > > >
> > > >-EH
> > > >
> > > >-----Original Message-----
> > > >From: Chuck Larrieu [mailto:[EMAIL PROTECTED]]
> > > >Sent: Wednesday, May 23, 2001 10:37 PM
> > > >To: [EMAIL PROTECTED]
> > > >Subject: ARP versus Proxy-arp [7:5664]
> > > >
> > > >
> > > >At the risk of becoming another Bob Vance......
> > > >
> > > >I'm reading Doug Comer's TCP/IP reference, on the
> > > assumption that it can't
> > > >hurt to really get into how TCP/IP works.
> > > >
> > > >Proxy-arp versus normal arp.
> > > >
> > > >A host does not know the physical address of
> > > another host so it sends out an
> > > >ARP request. If the host in question lies on
> > > another network, a router
> > > >responds to that request. Proxy ARP, correct?
> > > >
> > > >A host through it's TCP stack does the XOR and
> > > determines that a host lies
> > > >on another network. The host therefore sends the
> > > packet to the device
> > > >indicated as its default gateway in its
> > > configuration. It sends an ARP
> > > >request for the MAC of the default gateway. Normal
> > > ARP?
> > > >
> > > >So in other words, proxy arp may be viewed as
> > > something of an obsolete
> > > >protocol / operation in that most modern TCP stacks
> > > contain the mechanisms
> > > >for doing the network XOR determination, and then
> > > using the default gateway.
> > > >A modern stack would recognize that a host is on a
> > > different network and go
> > > >the default gateway route, so to speak.
> > > >
> > > >In other words, the necessity for proxy arp is
> > > eliminated for the most part
> > > >because of the default gateway concept and the
> > > modern TCP stack.
> > > >
> > > >Has it sunk through this thick head finally?
> > > >
> > > >PS Comer states that proxy arp is aka arp hack. :->
> > > >
> > > >Chuck
> > > >
> > > >One IOS to forward them all.
> > > >One IOS to find them.
> > > >One IOS to summarize them all
> > > >And in the routing table bind them.
> > > >
> > > >-JRR Chambers-
> > > >FAQ, list archives, and subscription info:
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> > > >Report misconduct and Nondisclosure violations to
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> > >
> > >
> > > ________________________
> > >
> > > Priscilla Oppenheimer
> > > http://www.priscilla.com
> > > FAQ, list archives, and subscription info:
> > > http://www.groupstudy.com/list/cisco.html
> > > Report misconduct and Nondisclosure violations to
> >[EMAIL PROTECTED]
> >
> >
> >__________________________________________________
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>
>
>________________________
>
>Priscilla Oppenheimer
>http://www.priscilla.com
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________________________
Priscilla Oppenheimer
http://www.priscilla.com
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