Doc suggestion - clarifying behaviour when routes are moving between protocols

[Mark Shuttleworth]

Hi folks

Thank you for Bird, I have enjoyed working with it over the holidays for a
personal project routing between offices. Along the way I got stuck on an
issue that might catch other people as well, so I thought to describe it
here in the hope that docs are updated to make the behaviour more clear.

I have a metropolitan area network between a few sites. Each of those sites
has its own internet connection, but the MAN is much faster, so my goal was
to route traffic to whichever exit point had the best bandwidth, using the
different sites as backup internet gateways. At each site, I was testing
the local connectivity (looking beyond simple link up to whether I could
reach common sites) and then, if the link was good, adding a static route
out from that location.

I wanted to use Bird to export and distribute those static routes to other
sites, with custom metrics to prioritise the gateways that had the best
bandwidth.

The expected behaviour for me was that a local, statically defined default
route would be imported and have the custom metric set based on its
bandwidth, and that the 'best' route would then win and be distributed by
OSPF to the other sites.

The problem I ran into was that the implicit protocol preference comparison
prevented the local route from even being considered at all. So once a
route had propagated to a site with Bird, it could not be displaced by a
'better' static route, because of the relative preferences.

I ended up with a config like this, where the explicit preference solved
the issue:

protocol kernel {
        learn;                  # Learn alien routes from the kernel
        ipv4 {                  # Connect protocol to IPv4 table by channel
                import filter {
                        if net = 0.0.0.0/0 then { # We only want default
routes added manually to this machine
                                ospf_metric2 = 1000;
                                preference = 150;
                                accept "Accepted default route from kernel";
                        }
                        reject;
                };
                export all;     # Install all Bird routes in kernel
        };
}

I couldn't find any documentation that explained the role of 'preference'
in the import process, so I was stuck on this for an embarrassing amount of
time :blush: In the end, this blog https://mk16.de/blog/bird-preference/
and some tangential comments in mailing lists led me to try the explicit
preference, and that solved the issue. Apologies if I missed this in the
docs, but if it isn't already covered, perhaps a more detailed explanation
of the way routes are compared for import into a table would help others.

Again, thank you for Bird! I think we might build it into Ubuntu in some
interesting ways :)

Mark
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