OSPF performance/SPF calculations

Ondrej Zajicek santiago at crfreenet.org
Sun Apr 25 23:20:33 CEST 2010

On Sun, Apr 25, 2010 at 06:08:04PM +0200, Joakim Tjernlund wrote:
> > 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.
> Our ptp links typically have the same cost between the same two routers so
> it is not unlikely that the link will be asymentric I think. However I
> don't think that will be a problem anymore as BIRD don't use/support
> equal cost multipath, right?


> Right, how about these statements from RFC 2328:
> These talk about inheriting ALL next hops from the parent and I don't
> see the BIRD does that. Looks like BIRD just selects one next hop
> from its parent?
> Assuming it is, does that impose any restricts?

Yes, BIRD chooses one next hop. Because BIRD does not implement equal
cost multipath, this is not a problem.

> Some other random questions:
> One think I always wanted is the possibility to add an host IP address to
> all areas, in my case I like to export the routers primary IP address
> to all areas so the router can always be reached. Is that possible with

You can use 'stubnet' option to add 'virtual' stub network (in that case
/32 stub) to the router-LSA. You can add this option to each area
config to get this in each area.

Or add /32 stub network to one area, other areas get a summary-LSA
with that address.

> If you pull the cable to an ethernet I/F that is currently in a OSPF
> domain, do BIRD delete the whole subnet from the R-LSA or do you
> leave a host route with the I/F's IP address in the R-LSA?

BIRD does not use link availability information, therefore if you pull
the cable, BIRD keeps the whole subnet in the router-LSA (if it is
a stub network).

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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