Advertise IPs as single regardless of prefix

[Alexander Zubkov]

Hi Anthony,

You mean you have for example ip address with mask configured on eth0:
2001:db8::cafe/64. And you want to have route 2001:db8::cafe/128 inside
bird without having to specify it manually in a static protocol for
example? And now you have to make a dummy interface with 2001:db8::cafe/128
configured on it?
AFAIK, you cannot do that automatically in bird (i.e. change network mask
of prefix). I thought if it would be possible to import single IP route
from "table local", Linux has such routes for IPs configured for the system:

local 2001:db8::cafe dev eth0 table local proto kernel metric 0 pref medium

But bird ignores routes with type local, so it would need some patching of
the source to do that.
Also IMHO, having IPs with overlapping prefixes should work well on Linux
if you know what you do.
You can also use such config, when you configure for
example 2001:db8::cafe/128 on eth0 and add device-route for 2001:db8::/64
on dev eth0. But that might not work for automatic configurations - DHCP,
SLAAC.

Regards,
Alexander Zubkov


On Tue, Sep 30, 2025 at 7:24 PM Anthony Hoppe via Bird-users <
bird-users at network.cz> wrote:

> Hello List!
>
> I am working on a project to bring L3 down to the individual host and
> am thinking through the transitionary configuration.
>
> For the purposes of this discussion, the hosts are assigned IPv4
> addresses within a /24 and IPv6 addresses within a /64.
>
> Currently, I'm using a dummy interface and adding the IPs they own as
> v4 /32 and v6 /128 and in combination with an export filter to only
> export /32 and /128 prefixes, this works great.
>
> However, I was wondering if there is a way to configure BIRD so that
> for the IPs configured on the host it supplants the configured prefix
> with /32 and/or /128 allowing me to eliminate the dummy interface?
>
> The dummy interface is mostly a transitory step as Linux (Debian, at
> least) will not let me assign a /32 or /128 to an interface where a
> larger overlapping prefix resides (which yes, I understand why you
> normally don't want to do this, haha).
>
> ~ Anthony
>
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