>From: TRON::"uunet!GreatCircle.COM!Firewalls-Owner" 6-OCT-1992 19:16:46.29
>To: jdw @
edu (Jeff Wandling)
>CC: firewalls @
>Subj: Re: cisco router & "interfaces used":"subnet" ratio
>Here's what is happening to your network, in all likelihood.
>You have configured things such that your hosts are not aware
>that your class B network is subnetted (mask 0xffff0000), while
>your router is aware of that (mask 0xffffff00).
> H1 H2
>Let's suppose hosts H2 and H3 are on different subnets.
>When H2 wants to communicate with H3, it sends an ARP
>request to get H3's Ethernet (or whatever your LAN is) address.
>H3 will respond to that, but also the router will step in and
>respond by giving H2 its own Ethernet address. This is called
>"proxy-ARP" and is done by the router when it sees that H2 and
>H3 are on different subnets. The end result is that packets
>between H2 and H3 are duplicated by the router and switched
>over the same cable.
This certainly isn't my understanding of how Cisco implements proxy arp!
The documentation and our observation indicates that the router will only
respond to an ARP when it believes it has the *best* route to that
destination. So in the case described with two hosts in different subnets
on the same cable, the Cisco should not reply.
>1. Your hosts to flip-flop between the true other host's Ethernet
> address and the router's Ethernet address in their ARP cache.
>Hope this wasn't too long.
>Cisco Systems, Europe
Westinghouse Electronic Systems Group
Internet: Adkins @