OSPF performance/SPF calculations

Ondrej Zajicek santiago at crfreenet.org
Fri Apr 23 16:01:16 CEST 2010

On Fri, Apr 23, 2010 at 03:27:10PM +0200, Joakim Tjernlund wrote:
> > This is an old code, which has some bugs i fixed a two weeks ago:
> Oh, I figured I cloned fairly recently. Must have jut missed you change.

I posted the change to the repository just two days ago.

> Anyhow I looked at the new code and it is an improvement but I think there
> is a flaw: It looks like the ptp code just find ANY interface that has a connection
> to the remote router, not the I/F that has created that particular RLSA entry.
> That may lead to asymmetric links, TX one link and RX on another.

This is not a problem because both SPF and calc_next_hop() chooses the
cheapest (full) ptp link. They both uses the same (local) metrics.

Even if you have asymetric metrics to have one link for one direction
and the other link for the other direction, the algorithm works,
because each side uses its metrics for both calculations.

There is one thing when the algorithm might break - if the cheapest full
ptp link is rejected by link_back() check. But such situation is just
transitional and would resolve immediately by either breaking the link
or receiving the new router-LSA from the neighbor.

Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."
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