Subject: [OSPF] Next hop calculation on PtP links over LAN

From: Ondrej Zajicek <[email-masked]>
Date: Fri, 31 Aug 2012 02:09:13 +0200

Hello

RFC 2328 does not specify how next hop should be computed on PtP ifaces
- it is supposed that a next hop IP is not needed [1], which is true for
physical PtP links, but not for PtP over LAN, as described in RFC 5309.
Although RFC 5309 acknowledges a need for next hop IP on such PtP links,
surprisingly it also does not specify how next hop should be computed in
OSPFv2 (if i did not miss someting).

To my knowledge, there are at least two currently used schemes:

1) Use value from Link Data field, in a same way of how RFC 2328
specifies next hop computation for Point-to-Multipoint links.
This scheme has several problems (e.g. does not work for unnumbered
links and there is no reliable way to match links from neighbor's router
LSA to my links if there are parallel links).

2) Use neighbor IP address from neighbor data structure of the neighbor on
given iface. This is probably more robust, but a bit strange, as neighbor
data structure is otherwise not used in routing table calculation.

Any comments on how next hop computation should be done in this case?

Slightly related question: Is it true that link-back check is broken in
OSPFv2 when there are parallel (unnumbered) links between two routers?
Footnote [23] says it is OK, but if router A has two links to router B,
while router B has just one link (in its router LSA), then router A during
routing table computation could select link that independently would
fail link-back check.

[1] RFC 238: "If the destination is a directly connected network, or a
router which connects to the calculating router via a point-to-point
interface, no next hop IP address is required."

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: [email-masked])
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."

---

From: Acee Lindem <[email-masked]>
Date: Fri, 31 Aug 2012 11:03:29 -0400

Hi Ondrej, 

See inline.  

> Any comments on how next hop computation should be done in this case?

Many layer 2 implementations support P2P ethernet as a true point-to-point topology. For P2P, my implementation uses #2 independent of whether the P2P is a true layer 2 P2P or a layer 3 emulation of P2P over a LAN topology. The RIB layer determines whether it is necessary. 

> OSPFv2 when there are parallel (unnumbered) links between two routers?

It is true that you cannot not unambiguously differentiate between the parallel unnumbered P2P links. However, for the unicast routing hop-by-hop paradigm, it shouldn't matter since we know the OSPF LSDBs should be synchronized and the IP routing tables should be consistent. For paradigm, e.g., traffic engineering, it is necessary to be able to uniquely identify the links. Note that no ambiguity exists in OSPFv3. 

I'd be interested in other opinions. 

Hope this helps,
Acee 

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