At 10:36 PM 5/23/01, Chuck Larrieu wrote:
>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?
Proxy ARP, yes. And I would add, "if the host in question lies on another
network that the router can reach...." I think the router must first make
sure it has a route to the network trying to be reached. Someone (Bob
Vance? ;-) can correct me if I'm wrong on that, but it makes logical sense.
It would be rude for the router to respond if it couldn't really offer
routing services to that network.
>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?
Normal ARP, yes. Isn't it an AND function, not XOR?
>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.
Proxy ARP came into existence because configuring TCP/IP was difficult. The
most common mistake was to forget to configure a default gateway. Also,
subnet masks caused grief for many people. In some cases Proxy ARP is a
workaround to a wrong subnet mask where stations thinking they are local
even though they aren't.
>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.
You make it sound like the default gateway concept is new. It's not new. It
existed before Proxy ARP.
Proxy ARP is still used, not only because users are still clueless, but
because network administrators have come to like it. You can set your
default gateway to your own IP address and Proxy ARP gets used also. That's
a somewhat new feature.
>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-
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