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

[Joakim Tjernlund <joakim.tjernlund at transmode.se>]

>
> 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?

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.

> 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
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