<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p><font face="Gentium">Hello!</font></p>
<p><font face="Gentium">TL;DR: That's currently not possible. But
there are other options.<br>
</font></p>
<div class="moz-cite-prefix">On 2024-01-10 00:28, Lukas Haase via
Bird-users wrote:<br>
</div>
<blockquote type="cite"
cite="mid:trinity-d4e4281a-8cc5-4636-a499-d31f6e732428-1704842891453@msvc-mesg-gmx025">
<pre class="moz-quote-pre" wrap="">Hi,
I have two "border gateways"; each establishes a BGP session to the outside world. An internal router connects to each of the border gateways and uses OSPF to exchange routes. (Note, other OSPF links may exist as well).
Both border gateways act as default gateways for the internal router. In order to create redundancy, each border gateway should send the default route via OSPF to the internal router. However, the default route should not be sent to any other device (otherwise it would mess up everything, incl tunnels).</pre>
</blockquote>
<p>First, if sending a default route through OSPF breaks stuff, you
should probably <span style="white-space: pre-wrap">filter that route out on these devices where it's unwanted. Just to be safe if somebody misconfigures something. And with that, you can then just send the default route to the whole OSPF.</span></p>
<p><span style="white-space: pre-wrap">Another approach would be filtering between areas in OSPF, which is quite fragile and not supported by BIRD anyway, but you can theoretically run two OSPF instances, one for the "with-default" network, and another one for the other network. This is very much a crazy setup which I do not recommend to go with unless you are sure that no better approach is available.</span></p>
<p><span style="white-space: pre-wrap">And finally, what may be even better, if your two uplinks are two different AS's … you can link the internal router to iBGP as well, balancing the traffic between the uplinks and failing to send all data over one uplink if the other fails.
</span></p>
<blockquote type="cite"
cite="mid:trinity-d4e4281a-8cc5-4636-a499-d31f6e732428-1704842891453@msvc-mesg-gmx025">
<pre class="moz-quote-pre" wrap="">
Additionally, injecting the default route should be contingent on having the BGP session established successfully (only then I can assume the route to the internet is good).</pre>
</blockquote>
<p>In BIRD v2, there will be a possibility to do prefix aggregation
added this year, and it looks like the configuration can be bent
enough to actually generate "just the default route" from it. Not
supported for now.</p>
<p>In BIRD v3, there will be more possible methods to generate
default route, anyway, no one of them is merged even into
thread-next yet.</p>
<p>I hope that I have at least pointed you the right way.</p>
<p>Have a nice day!<br>
Maria<span style="white-space: pre-wrap">
</span></p>
<pre class="moz-signature" cols="72">--
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.</pre>
</body>
</html>