Subject: [OSPF] Question about Stub advertisement of a PtP link RFC 2328

From: "Juergen Arlt" <[email-masked]>
Date: Tue, 28 Jul 2009 11:03:40 +0100

I have a very specific problem on an unnumbered point to point link for
an IPsec tunnel. This link is associated to another IP interfaces
address on the device to borrow an IP (source) address for the packets.
The link itself (as being unnumbered has no IP subnet assigned).

Which option of the following in "12.4.1.1.  Describing point-to-point
interfaces" would apply for the stub area advertisement? 

		     In addition, as long as the state of the interface
                    is "Point-to-Point" (and regardless of the
                    neighboring router state), a Type 3 link (stub
                    network) should be added. There are two forms that
                    this stub link can take:

                    Option 1
                        Assuming that the neighboring router's IP
                        address is known, set the Link ID of the Type 3
                        link to the neighbor's IP address, the Link Data
                        to the mask 0xffffffff (indicating a host
                        route), and the cost to the interface's
                        configured output cost.[15]

                    Option 2
                        If a subnet has been assigned to the point-to-
                        point link, set the Link ID of the Type 3 link
                        to the subnet's IP address, the Link Data to the
                        subnet's mask, and the cost to the interface's
                        configured output cost.[16]

I would read that option 2 would not apply as the link is unnumbered
therefore no subnet has been assigned to that link (even though an
associated address is set). 
For my specific PtP case the neighbor address is known as the tunnel
endpoint is manually configured (though not in any local network)
therefore I can use this for the Stub entry. 

Is that reading correct?

What if we are talking about a virtual link? 

        Although a virtual link acts like an
        unnumbered point-to-point link, it does have an associated IP
        interface address.  This address is used as the IP source in
        OSPF protocol packets it sends along the virtual link, and is
        set dynamically during the routing table build process.

Is has no known neighbor router IP as it knows only the neighbor routers
router-ID (not IP) and it has no subnet assigned to the virtual link
(though an associated IP). How should the stub advertisement look like -
which of the options apply?

Regards
Juergen Arlt

-------------------------------------------------
Juergen Arlt
Nortel GmbH
Senior Network Solutions Engineer
Global Network Technical Support
Mittlerer Pfad 26
70499 Stuttgart
Germany

Tel: +49 (711) 1394361
ESN: 595 4361
Fax: +49 (711) 1394330
-------------------------------------------------

---

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 28 Jul 2009 12:15:10 +0200

>
> I have a very specific problem on an unnumbered point to point link for an
> IPsec tunnel. This link is associated to another IP interfaces address on the
> device to borrow an IP (source) address for the packets. The link itself (as
> being unnumbered has no IP subnet assigned).
> Which option of the following in “12.4.1.1.  Describing point-to-point
> interfaces” would apply for the stub area advertisement?
> I would read that option 2 would not apply as the link is unnumbered therefore
> no subnet has been assigned to that link (even though an associated address is set).
> For my specific PtP case the neighbor address is known as the tunnel endpoint
> is manually configured (though not in any local network) therefore I can use
> this for the Stub entry.
> Is that reading correct?

There was a discussion about this some months ago(started by me) on this subject.
The short answer is that for unnumbered links you don't send Option 1 nor Option 2. The
spec is a bit unclear but that is what the list concluded.

> router-ID (not IP) and it has no subnet assigned to the virtual link (though
> an associated IP). How should the stub advertisement look like - which of the
> options apply?
> Regards
> Juergen Arlt
> _______________________________________________
> OSPF mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/ospf

---

From: "Juergen Arlt" <[email-masked]>
Date: Tue, 28 Jul 2009 11:22:44 +0100

As I need to convince our development team - can you give me some more
background on this. They are advertising currently the associated
address for the link (that to me is the incorrect approach) which caused
some misbehavior in the network.
Thanks and regards
Juergen

-----Original Message-----
From: Joakim Tjernlund [mailto:[email-masked]] 
Sent: Dienstag, 28. Juli 2009 12:15
To: Arlt, Juergen (GERST:476S)
Cc: [email-masked]
Subject: Re: [OSPF] Question about Stub advertisement of a PtP link RFC
2328

> I would read that option 2 would not apply as the link is unnumbered
therefore
> For my specific PtP case the neighbor address is known as the tunnel
endpoint
> is manually configured (though not in any local network) therefore I
can use

There was a discussion about this some months ago(started by me) on this
subject.
The short answer is that for unnumbered links you don't send Option 1
nor Option 2. The
spec is a bit unclear but that is what the list concluded.

> router-ID (not IP) and it has no subnet assigned to the virtual link
(though

---

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 28 Jul 2009 12:34:49 +0200

> some misbehavior in the network.

Just find the discussion in the archive. It was quite clear what to do.
Acee Lindem, chair of OSPF WG, clarified this and there was no
room for other interpretations. If you read the OSPF spec there are
lots of hints and the previous version of the OSPF spec was very clear
too.

 Jocke

---

From: Acee Lindem <[email-masked]>
Date: Tue, 28 Jul 2009 08:50:05 -0400

Hi Joakim, Juergen,

That is correct, there is no stub link at all associated with an  
unnumbered link. It sounds like you have some requirements to  
advertise local endpoints though. If you are truly modeling the  
tunnel as an unnumbered P2P interface, then these requirements should  
be satisfied by advertising the endpoint interface independently as a  
stub link. Many times this is the loopback and the endpoint for many  
tunnels.

Thanks,
Acee
On Jul 28, 2009, at 6:34 AM, Joakim Tjernlund wrote:

>>> IPsec tunnel. This link is associated to another IP interfaces  
>>> address
>> on the
>>>                         address is known, set the Link ID of the  
>>> Type
>> 3
>>> an associated IP). How should the stub advertisement look like -  
>>> which
>> of the
>>> options apply?
>>> Regards
>>> Juergen Arlt

---

From: "Juergen Arlt" <[email-masked]>
Date: Tue, 28 Jul 2009 14:09:49 +0100

I don't want to worm up the long discussion that took place in January
(thanks to Joakims hint) but I still read that the option 1 should be
used to advertise the remote router IP as a stub host route - though it
is not required for the protocol to work. I agree however that in the
LSA example of 2328 Router 3 ommited the option 1 as well.

But the advertisement of the neighbor IP is clearly stated in RFC 1583
and I would read now the same in 2328.

Quote "When interface addresses are assigned, they are modelled as stub
links, with each router advertising a stub connection to the other
router's interface address."

The issue however is actually not the tunnel endpoint reachability but
our developers did advertise the network of the unnumbered associated IP
address as stub (which did not even have OSPF configured) along with the
ptp link which led to some weird routing problems. 

Now I am trying to convince them to correct this and want to make sure
it is done correctly. So I proposed to use option 1 - as we know the
tunnel endpoints IP (manual configuration) and remove the incorrect
option 2 implementation.

Thanks
Juergen

-----Original Message-----
From: Acee Lindem [mailto:[email-masked]] 
Sent: Dienstag, 28. Juli 2009 14:50
To: Joakim Tjernlund
Cc: Arlt, Juergen (GERST:476S); [email-masked]
Subject: Re: [OSPF] Question about Stub advertisement of a PtP link RFC
2328

Hi Joakim, Juergen,

Thanks,
Acee

---

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 28 Jul 2009 17:20:39 +0200

> router's interface address."

Since the P2P I/F has an IP address(as I read it) and I guess an /32 mask
I think you can view it as an numbered I/F and use Option 1.
It does not matter that the IP address isn't unique I think.
I you have many P2P links you get a significantly bigger Router LSA
though so I think Acee's suggestion is better.

---

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 28 Jul 2009 19:16:52 +0200

> router's interface address."

BTW, I read "interface addresses are assigned" == numbered so this
para does not apply to unnumbered I/F's

 Jocke

---

From: Acee Lindem <[email-masked]>
Date: Wed, 29 Jul 2009 04:28:38 -0400

Hi Juergen,
I agree that in this situation you should not advertise the subnet of  
the outgoing link for the IPsec tunnel. If you advertise the tunnel  
endpoint as per option 1, you are not modeling the link as an  
unnumbered link. The OSPF specification does not specify how to model  
the tunnel - this is up to your implementation.
Acee
On Jul 28, 2009, at 9:09 AM, Juergen Arlt wrote:

>>>>                         to the subnet's IP address, the Link  
>>>> Data to
>>> the
>>>> is manually configured (though not in any local network)  
>>>> therefore I
>>> can use
>>>> this for the Stub entry.
>>>> Is that reading correct?
>>>