Re: Unnumbered and Virtual Links

On Sat, Jul 17, 2010 at 09:31, Sonu Khandelwal (sokhande)
<sokhande_at_cisco.com> wrote:
> Hi Marko,
> R4 (4.4.4.4/32) and R5 (5.5.5.5/32) are not on common subnet on interface . I am wondering how is neighbor ship coming up between.

Well, this is a little bit tricky to understand ;-). First off, if you
configure IPs directly on the interface, neighbors will not form. If
you enable "debug ip ospf adj", you will see message like this one:

OSPF: Rcv pkt from xx.xx.xx.xx, Serial0/0/0, area 0.0.0.0 : src not on
the same network

Now, the reason for that is that address/mask don't match. However,
when we configure IP unnumbered, these IP addresses are not sent in
hello packets. Here's the quote from section 9.5 of RFC 2328:

> 9.5. Sending Hello packets
> [...]The Hello packet also contains the IP address mask of the attached network (Network Mask).
> On unnumbered point-to-point networks and on virtual > links this field should be set to 0.0.0.0.[...]

So, when your router receives hello packet with mask 0.0.0.0, it knows
that other end is using unnumbered link and will not perform
verification of the IP/MASK.

--
Marko Milivojevic - CCIE #18427
Senior Technical Instructor - IPexpert
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