Exporting a larger prefix if a smaller prefix is being exported

[Lukas Haase]

Hi Alex,

> Gesendet: Samstag, 13. Januar 2024 um 06:31 Uhr
> Von: "Alexander Zubkov" <green at qrator.net>
> An: "Lukas Haase" <lukashaase at gmx.at>
> Cc: bird-users at network.cz
> Betreff: Re: Exporting a larger prefix if a smaller prefix is being exported
>
> Hi,
> 
> You cannot do "direct" prefix aggregation to a lager prefix in Bird
> yet. But there are some ways to workaround it. You can define a static
> route with recursive nex-hop like 192.0.2.x, and filter it out when it
> is not reachable, but for any subprefix in /24 you would need to
> define 256 of such static routes. So it is up to you how practical it
> is.

Interesting idea, this would be practical for me but I do not completely understand yet what you mean.
Which routes would I define and what would be the next hop ("like 192.0.2.x")?

As an example, suppose the following prefixes are in my routing table and are directly or indirectly reachable: 192.0.2.208/28, 192.0.2.250/31, 192.0.2.184/29, 192.0.2.254/31, 192.0.2.176/29.

Are you proposing?

protocol static prefix_aggregation
{
  route 192.0.2.0/24 via 192.0.2.209;
  route 192.0.2.0/24 via 192.0.2.250;
  route 192.0.2.0/24 via 192.0.2.285;
  route 192.0.2.0/24 via 192.0.2.254;
  route 192.0.2.0/24 via 192.0.2.177;
}

If so, how do I avoid that 192.0.2.0/24 will be exported five times?
And how do I set up an export filter on "next-hop is not reachable"?

By the way, the sub-prefixes, would I just export via a filter like this?

export filter {     
    if (net ~ [192.0.2.0/24{25,32}]) then { 
        accept;
    }
    reject;
}

Thanks,
Luke 



> You can also make some external daemon watching your kernel routes
> and adding/deleting the aggregate route to the table.
> 
> Regards,
> Alexander
> 
> On Sat, Jan 13, 2024 at 2:05 AM Lukas Haase via Bird-users
> <bird-users at network.cz> wrote:
> >
> > Hi,
> >
> > Is is somehow possible to export a larger prefix if one or more sub-prefixes (subnets) are exported ... but also remove that prefix if no smaller subnet exist any more?
> >
> > Example: As soon as 192.0.2.44/32 or 192.0.2.208/28 (or any other prefix inside 192.0.2.0/24) is exported via eBGP, also export prefix 192.0.2.0/24. If no sub-prefixes are left, also remove 192.0.2.0/24 from export.
> >
> > Background for my question is BGP. As is well known, the smallest prefix I can announce over eBGP is /24. I use bird as a border gateway and I announce various smaller prefixes via iBGP. The smaller prefixes will take precedence in my peering neighboring AS but the /24 is required to announce my network farther out.
> >
> > But why would I want that? Because there are actually two border gateways. If all internal links to one of these gateways breaks, the full subnet should not be announced any more (otherwise the traffic would be dropped). If at least one subnet is announced, I assume that the internal mesh is strong enough to find its way.
> >
> >
> > Thanks,
> > Luke
> >
> >
>