The use of the "ip ospf network point-to-multipoint non-broadcast" command
is really restricted to supporting some very select environments. The
explanation I was given when I enquired was that it was brought out by Cisco
to support RFC 1577 Classical IP over ATM scenarios.
RFC 1577 Classical IP over ATM requires the use of routed PDU encapsulation
as per RFC 1483. This precludes broadcast support as there is no traditional
Data Link Layer information used in the encapsulation. The IP datagram is
encapsulated directly into ATM cells using AAL 5. This requires the mapping
of IP addresses to ATM addresses, which is handled by an ARP service,
normally configured on a switch or ATM end station. That device receives
registration requests and ARP requests and serves up address mappings to
clients. The important thing here is that this environment is completely
broadcast (and hence multicast) free, because there is no support for
anything other than unicast.
Now try running OSPF over that! The OSPF point-to-multipoint mode supports
multicast and the routers will attempt to discover one another using IP
multicast, except it will never work over a Classical IP network, hence the
need to provide for a workaround. That's why Cisco brought out the OSPF
point-to-multipoint non-broadcast mode. I can't imagine that anyone is still
using it, because if you're still using ATM, the likelihood is that you've
moved with the times and have been implementing LANE/MPLS etc., and that
very definitely will support multicast.
--
Edward Solomon
CCNP, CCSI
Technical Instructor
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""John Neiberger"" wrote in message
news:[EMAIL PROTECTED].;
> I've been reading the CCO configuration guides and I now have a
> question. What is the difference between the following:
>
> ip ospf network point-to-multipoint
>
> and
>
> ip ospf network point-to-multipoint non-broadcast
>
> ??
>
> More specifically, I really want to know when you would use the latter
> command. The example given on CCO is if we have excluded the broadcast
> keyword from a frame relay map. I don't think I've ever seen anyone
> configure a frame map without that keyword so I don't know why you'd
> ever want to do that.
>
> Any thoughts here?
>
> Thanks,
> John
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=37259&t=37205
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