Subject: [OSPF] multilple numbered PtoP links with the same local IP address

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 11 Aug 2009 21:12:59 +0200

Is there something in the OSPF spec which prevents it from operate
on multiple PtoP interfaces with the same local IP address?

I have that working, but there are arguments that this is allowed by
OSPF, i.e, ethier use unnumbered links or unique local IP addresses.

 Jocke

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From: "Sean Andersen" <[email-masked]>
Date: Tue, 11 Aug 2009 13:41:27 -0700

You should use unique IP addresses or unnumber them to another interface
that is up all the time (e.g. loopback, or Ethernet) and make sure that ospf
is running on the interface (loopback or ethernet) that you are using for
unnumbered (do not unnumber one ptp to another ptp)

Sean

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Joakim Tjernlund
Sent: Tuesday, August 11, 2009 12:13 PM
To: [email-masked]
Subject: [OSPF] multilple numbered PtoP links with the same local IP address

 Jocke

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From: Joakim Tjernlund <[email-masked]>
Date: Tue, 11 Aug 2009 23:04:40 +0200

> unnumbered (do not unnumber one ptp to another ptp)

Why do I have to? What you describe is common practise but is not the
same as one isn't allowed to.

 Jocke

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From: Joakim Tjernlund <[email-masked]>
Date: Thu, 13 Aug 2009 09:40:45 +0200

> on multiple numbered PtoP interfaces with the same local IP address?
>

Got some comments on this but nothing conclusive, I really want
to know. Acee, what do you think?

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From: Acee Lindem <[email-masked]>
Date: Thu, 13 Aug 2009 20:54:30 -0400

> to know. Acee, what do you think?

Since there is only a type 1 (router link) in the Router-LSA for each  
unnumbered link, it makes no difference how many use the same IP  
address.

The only OSPF protocol restriction is that you must advertise at  
least one intra-area IP address (via a type 3 link) in any transit  
area you want to be the endpoint for a virtual link.

Hope this Helps,
Acee

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From: Acee Lindem <[email-masked]>
Date: Thu, 13 Aug 2009 18:13:48 -0400

Hope this Helps,
Acee

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From: Joakim Tjernlund <[email-masked]>
Date: Fri, 14 Aug 2009 09:14:11 +0200

> unnumbered link, it makes no difference how many use the same IP

Right, but how about multiple NUMBERED PtoP interfaces with the
same local IP address? Then there will be multiple identical
type 3 too.

 Jocke

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From: Acee Lindem <[email-masked]>
Date: Fri, 14 Aug 2009 10:23:15 -0400

On Aug 14, 2009, at 3:14 AM, Joakim Tjernlund wrote:

> same local IP address?

I'd argue that if your configuration supports this, all but one of  
the interfaces is unnumbered.

> Then there will be multiple identical
> type 3 too.

Even though it is non-standard, I don't see any problems. I believe  
Dave alluded to the fact that some OSPF implementations may have ABR  
SPF optimizations which can have problems when the same address is  
advertised in multiple addresses - I know fixed some problems here in  
one implementation.

Acee

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From: Joakim Tjernlund <[email-masked]>
Date: Fri, 14 Aug 2009 17:03:23 +0200

> the interfaces is unnumbered.

hmm, please clarify. I do mean treating each and every PtP
as a numbered PtP link. Is there something that prohibits
this in the spec? Of course not all routers will
be able to handle this but in that case it would be a
limitation in the impl.

Is it allowed to have numbered on one side of the link
and unnumbered on the other side?

> advertised in multiple addresses - I know fixed some problems here in

Yeah, when one only uses the router LSAs to identify the interface
you are in trouble.

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From: Acee Lindem <[email-masked]>
Date: Fri, 14 Aug 2009 13:11:33 -0400

> and unnumbered on the other side?

Yes - this is fine.

Thanks,
Acee

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From: Anton Smirnov <[email-masked]>
Date: Sat, 15 Aug 2009 14:47:08 +0200

>> the interfaces is unnumbered.

   Joakim,
   IP address must be unique and assigned to one numbered IP interface
only. This has nothing to do with OSPF, it is requirement of a basic IP
addressing paradigm and covered in relevant IPv4 architecture documents
(no, I don't have a reference ready).
   This 'limitation' is the reason why unnumbered interfaces were
invented in the first place. If "multiple NUMBERED PtoP interfaces with
the same local IP address" worked well then nobody would bother to have
unnumbered interfaces.

   Note that numbered interface by definition implies the interface has
mask <32 bits and that remote end has IP address from the same IP
subnet. If implementation allows configuration of IP address with mask
of 32 bits or no mask at all then this in fact is just implementation of
IP unnumbered interface where 'master' interface which owns the IP
address is not explicitly shown. Conceptually it would be IP unnumbered
nonetheless and should be treated by OSPF accordingly.

Anton

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From: Joakim Tjernlund <[email-masked]>
Date: Sat, 15 Aug 2009 15:07:08 +0200

> nonetheless and should be treated by OSPF accordingly.

This is the case I am thinking on(numbered with mask=32), but I am treating it
as a numbered interface in some cases and unnumbered in others and it works
very well. The only difference OSPF makes on these two is adding Option 1
in the router LSA for the numbered one and unnumbered sends ifindex.

Perhaps this conceptually a unnumbered I/F but one do not have to advertise
it as such to get a working OSPF domain.

 Jocke