Re: OSPF Router ID & Network Types

Hi Nish,

I haven't read that RFC cover-to-cover in quite some time but I think
this probably has to do with unnumbered serial interfaces (once a
fairly common practice but I personally don't see it much anymore).
Notice that the below passage does not actually say that the point-to-
point *network* itself is identified by a RID, but rather that the
*sender* of a *hello* is identified by such. The other part of the
passage you quoted goes as follows:

//
If the packet type is Hello, it should then be further processed by
the Hello Protocol (see Section 10.5). All other packet types are sent/
received only on adjacencies. This means that the packet must have
been sent by one of the router's active neighbors. If the receiving
interface connects to a broadcast network, Point-to-MultiPoint network
or NBMA network the sender is identified by the IP source address
found in the packet's IP header.
//

And then the below-quoted part about using the RID found in the *OSPF*
header to identify a sender in the case of the point-to-point network
type (vs. the source address found in the *IP* header for the other
network types). OSPF does a lot of sanity checking - including on
received hellos - and unnumbered interfaces tend to complicate some of
that logic. I believe this is one of the countermeasures taken to
address some "under-the-hood/bonnet" conflicts that could otherwise
result if an OSPF speaker were to try to identify the sender of a
hello by its source IP address in the case of point-to-point networks,
which may or may not involve unnumbered interfaces.

Regards,

Scott

On Jan 21, 2011, at 12:12 , Nish Vamadevan wrote:

> Hi,
>
> As I am reading through RFC 2328, Came up with this statement and I
> can't
> seem to come up with a logical explanation, why... Maybe someone can
> help me
> out?
>
> When it comes to the Hello Packet,
>
> //
> If the receiving interface connects to a point-to-point network or a
> virtual
> link, the sender is identified by the Router ID (source router)
> found in the
> packet's OSPF header.
> //
>
> I can understand the part about the virtual-link because it is
> created using
> the Router ID.
>
> What I can't understand is why a point-to-point network is
> identified by the
> Router ID. And having said that, Point-To-MultiPoint isin't. (It
> also states
> that, as we know Point-To-MultiPoint is a collection of point-to-
> point)
>
> I'd appreciate it if anyone can shed some light in this...
>
> Thanks,
>
> --
> Regards,
> Nish
>
> www.twitter.com/nish
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Fri Jan 21 2011 - 14:10:16 ART