Path: utzoo!attcan!uunet!lll-winken!ames!pasteur!ucbvax!NSIPO.ARC.NASA.GOV!medin
From: medin@NSIPO.ARC.NASA.GOV ("Milo S. Medin", NASA ARC NSI Project Office)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Routing with redundant connections
Message-ID: <8905120850.AA01162@nsipo.arc.nasa.gov>
Date: 12 May 89 08:50:41 GMT
References: <8905120818.AA27749@ames.arc.nasa.gov>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 47


Roger, No redirects will be sent by the routers to the hosts since the next 
hop will not be on the same net (ethernet B).  

The ARP's that will occur won't change the next hop.  With some effort, and
possibly breaking the hosts view of what the real subnet mask is on the B
cable, proxy arp might work, but I think it's a gross hack in this
case and you'd be better off without it.

When OSPF comes along, it's not really any different, but you can instead
run full OSPF routing on Ethernet B, and if you do that, you'll get the
redirects as you want.  This isn't a big deal, since most of the time you go
to a 2 net structure like you describe is to reduce broadcast traffic
on a LAN with hosts on it, or because you worry about some host there injecting
foul routing information in the system.

OSPF addresses these problems by Multicasting on broadcast LAN's, like Ethernet,
and thus hosts won't hear it unless they are a part of that multicast
group.  Also, misconfigured hosts won't be sending back ICMP noise at the routers
since they won't be hearing the multicasts.  It also means that even if they
run gated or RIP, they still won't be having to digest full routing info.
That way you can use RIP on the cable to lead hosts to a default router,
and run OSPF between the routers with full routing info flowing via
multicasts...

Even if a 'enemy' OSPF router were present on that cable, he
still wouldn't be able to inject unwanted routing info into the system
because OSPF supports authentication across all router adjacencies.  Thus
the 'enemy' router won't be believed by router 1 or router 2.  And thus you 
won't have coincidentally colocated routers fouling your routing domain.

If you are worried about loading on ethernet B, OSPF will also allow you
to load balance across ethernet A and B (equal cost multipath support is built
in to the protocol, though a particular router implementation (e.g. BSD Unix)
may not implement it).  If you are load balancing across A and B, then
one of the next hops will be on the same net, and the routers should issue
redirects as appropriate...  OSPF is most topologies will generate a lot
less routing traffic than RIP anyway.

Even if you don't like SPF routing algorithms, OSPF's multicasting and
router authentication are really nice things to have.  It's too bad it's
taken this long for a standard IP routing protocol to incorporate these
features.  The technology to do it has been with us for some time.  Better 
late than never though!

						Thanks,
						   Milo