Re: Unnumbered and Virtual Links

Marko

Did you happen to get the virtual link working in the topology I provided?

On Sat, Jul 17, 2010 at 2:50 PM, Marko Milivojevic <markom_at_ipexpert.com> wrote:
> 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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Received on Sat Jul 17 2010 - 17:22:16 ART