This is all new information to me! I had no idea that I could place two
separate serial interfaces into the same subnet, so I just tried it. I set
up two back to back cables between two routers and put both lines on the
same subnet. I was amazed that it didn't freak out.
I noticed that when I would ping from one to the other the router did
per-packet load balancing regardless of 'ip route-cache' setting, which
makes sense. This really doesn't have anything to do with route caching or
fast switching.
Now I just tried running OSPF across these links and the routers didn't
appear to be confused. They each see the other as being adjacent on two
different interfaces, but they don't really care. They just put both routes
into the routing table.
I suppose this is no different than if I were to configure a loopback
interface on each side and then use ip unnumbered on both links. In that
case they'd also be in the same subnet.
Interesting, thanks for pointing this out to me!
John
> I'll be quite honest and say I haven't done a detailed investigation
> of the IOS implementation restrictions here. My intuition would be
> that IOS has one ARP cache per subnet per physical router, and having
> multiple router ports in the same broadcast subnet confuses the ARP
> mechanism. On a first scan of RFC 1812, I don't see any inherent
> architectural limitation on more than one interface in a subnet on a
> physical router.
>
> I suspect the reason that multiple serial interfaces can work is that
> they don't routinely ARP, since they don't have to resolve MAC
> addresses they don't have.
>
> >However, just in the last few days, we read on this list that serial
> >interfaces can have 2 or more in the same subnet. I think one poster
said
> >there was a maximum of 6?
>
> Which would be consistent with the maximum interfaces in load
> sharing. I suspect there are some IOS internal games here, where an
> (internal only) virtual interface describes the next hop for a
> destination to which there is load sharing, and some type of
> recursion takes place (perhaps interacting with a cache) to decide
> which physical interface to use.
>
> These are guesses, however.
>
> >
> >But ethernet interfaces cannot share a subnet.
> >
> >Kevin Wigle
> >
> >----- Original Message -----
> >From: "John Neiberger" <[EMAIL PROTECTED]>
> >To: <[EMAIL PROTECTED]>
> >Cc: <[EMAIL PROTECTED]>
> >Sent: Friday, 30 March, 2001 10:41
> >Subject: Re: Stupid question
> >
> >
> >> This isn't a stupid question, it's a very important point to make.
If
> >> you are routing, each interface on the router must be in its own
subnet.
> >> Otherwise routing would not work. If you're bridging, then the
bridged
> >> interfaces are in the same subnet but you don't specifically assign
an
> >> IP address to those interfaces.
> >>
> >> I'm guessing that you're really asking the former question: in a
> >> routing situation can two different interfaces be in the same subnet,
> > > and the answer is no.
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