Dan Swartzendruber wrote:
>
> On Tue, 5 Jun 2001, Pierre Fortin wrote:
>
> >
> > Assuming the routers are there to access Net[AB], you can turn on proxy ARP as
> > Nathan suggested in his reply to simplify other host configuration requirements
> > and reduce unnecessary router hops and resultant ICMP redirects.
> >
> > Proxy ARP -- a short course: when a host ARPs for a remote destination without
> > trying to go thru a GW, a router which knows how to get to that destination will
> > Proxy ARP reply allowing the host to send its packets to what it thinks is the
> > destination (hence "proxy"). Note that a Proxy ARP reply is no guarantee of the
> > best route, just a viable route; but in your case, unless the topology is more
> > complex, only the best router will reply since the other router would have to
> > route packets back out the same interface they come in on... not what routers
> > are 'trained' to do...
>
> i guess. i really don't like doing proxy arp, and it's almost never
> necessary.
Welll... there are some choices (a sampling):
1) define a gateway in all hosts. When a host wants to get to a remote host,
it finds the gw in its table and ARPs for the gw, then sends the packets to the
gw.
2) don't define gw in hosts. Let them ARP for the destination and any
router(s) which knows how to get there (without routing back over the incoming
interface) will respond.
Now, lets look at some potential problems:
In 1), what happens when the gw dies? What if there is an alternate gw? Are
the gws configured to backup each other in the event one fails? If so, the
backup router must take over the failing router's IP address, and maintain its
own...
In 2), the slowest ARP reply wins; in certain topologies, this can be extremely
detrimental to traffic (we wrote an ARP responder circa 1988 so that a
promiscuous server could late (~500ms) ARP-reply hosts with the proper proxy
router's MAC). However, it simplifies host configs for alternate routing.
[Proxy] ARP is local only; but it can help simplify some network configuration
issues...
There is no hard and fast rule for all networks; just a lot of reasoned
compromises...
Pierre
PS: Yes I have negative opinions on certain protocols; but unlike proxy ARP,
those "deserve" it.. :^D
