Re: Re: Exporting a larger prefix if a smaller prefix is being exported

15 Jan 2024


      On Mon, Jan 15, 2024, 10:15 Lukas Haase <lukashaase@gmx.at> wrote:
...
Hi Alexander,
Thank you so much!
Debian has bird2 package as well so I migrated and indeed it works!!
There must be a bug in bird1 that causes recursive routes in static
protocol not to resolve...
...
As just a minor change to your solution, I changed the export filter in
the bgp protocol to:
if(net ~ [192.0.2.0/24{24,32} <http://192.0.2.0/24%7B24,32%7D>]) then
accept;
because I actually want to export the sub prefixes AND the /24.
Sure, if you need those. It is slightly out of scope of the topic, so I did
not included them in my filter. :) BTW ”x.x.x.x/24{24,32}” id equivalent to
”x.x.x.x/24+”.
...
Sorry, there is just one last thing: Currently I define my /24 like this:
protocol static static_bgp
{
  ipv4;
  route 192.0.2.0/24 via Public-Ip-As-Seen-By-Peer;
}
How do I make sure that whatever route is sent has the public IP as
next-hop? Since I am using recursive now, I do not have control over it.
Will this be set automatically to what I set as "source address" in the
bgp peer definition?
Or should I explicitely add "next hop self"? Or should I explicitly modify
"bgp_next_hop = ..." in the export filter of the bgp peer?
You can check the docs for the defaults. "next hop self" might be the
default for eBGP, but I do not remember exactly. You can set it explicitly
also. And yes, you can either use this option or override the next hop in
the effort filter.
...
Thanks,
Luke
...
Gesendet: Montag, 15. Januar 2024 um 00:11 Uhr
Von: "Alexander Zubkov" <green@qrator.net>
An: "Lukas Haase" <lukashaase@gmx.at>
Cc: bird-users@network.cz
Betreff: Re: Exporting a larger prefix if a smaller prefix is being
exported
Hi,
I cannot tell for bird1, unfortunately. It might not work there at
all. Here is working example for bird2, I tested it and it seems
valid. I export smaller routes to a separate table, so that static
protocol use only those routes for recursive resolution, otherwise it
will also try to use default route and itself too. I also put static
routes to a separate table and pipe from it and filtering out the
unreachable ones. Because otherwise an unreachable route could be
selected the best (they all have the same metric) and not exported to
bgp. So check for unreachable in bgp export does not work well
actually, if there are more than one static route to choose from.
For bird1 please note, that it is announced EOL already. And it is
highly recommended to migrate to bird2.
ipv4 table aggr_src;
ipv4 table aggr;
protocol bgp test
{
        local as 64512;
        source address 10.55.55.251;
        ipv4 {
                import none;
                export filter {
                        if(net = 192.0.2.0/24) then accept;
                        reject;
                };
        };
        neighbor 10.55.55.250 as 65534;
        password "xyz";
        multihop 2;
}
protocol static prefix_aggregation
{
        ipv4 { table aggr; };
        igp table aggr_src;
        route 192.0.2.0/24 recursive 192.0.2.207;
        route 192.0.2.0/24 recursive 192.0.2.208;
        route 192.0.2.0/24 recursive 192.0.2.209;
}
protocol device {}
protocol direct { ipv4; }
protocol pipe pipe_aggr_src
{
        table master4;
        peer table aggr_src;
        export filter {
                if (net ~ [192.0.2.0/24{25,32}
<http://192.0.2.0/24%7B25,32%7D>]) then accept;
                reject;
        };
        import none;
}
protocol pipe pipe_aggr
{
        table master4;
        peer table aggr;
        export none;
        import where dest != RTD_UNREACHABLE;
}
On Mon, Jan 15, 2024 at 8:59 AM Lukas Haase <lukashaase@gmx.at> wrote:
...
Hi Alexander
...
Gesendet: Sonntag, 14. Januar 2024 um 23:03 Uhr
Von: "Alexander Zubkov" <green@qrator.net>
An: "Lukas Haase" <lukashaase@gmx.at>
Cc: bird-users@network.cz
Betreff: Re: Re: Exporting a larger prefix if a smaller prefix is
being exported
...
...
Hi Lukas,
Two questions.
You add dummy interface on another node that propagates it via ospf
to
...
your border?
I do not, it's added locally. Same machine as bird is running. I just
need
interface "dum0" { stub yes; };
to bring it into bird's routing table. I could also use protocol
"direct" but to simulate it as close as possible to my actual use case
(where the possible smaller prefixes will come over OSPF) I wanted to use
OSPF.
But in my understanding, it shouldn't matter where the route comes
from.
...
And the most important one - you use bird version 1?
Yes, it's the version included in Debian/Ubuntu stable. As long as I
am not hitting a huge roadblock I'd really like to keep it.
Thanks,
Luke
...
On Mon, Jan 15, 2024 at 6:23 AM Lukas Haase <lukashaase@gmx.at>
wrote:
...
Hi Alexander,
Thank you again, this is really promising and I think I get the
gist of it.
...
I have just one issue left: The /24 prefix keeps showing as
unreachable.
I have set up a test peer (both sides) to verify that routes come
through.
Here is my config:
protocol bgp test
{
        local as 64512;
        source address 10.55.55.251;
        import none;
        export filter {
                # announce all of our sub-prefixes
                if(net ~ [192.0.2.0/24{25,32}
<http://192.0.2.0/24%7B25,32%7D>]) then accept;
# if at least one of them is reachable, announce
the entire /24
                if(net = 192.0.2.0/24 && dest != RTD_UNREACHABLE)
then accept;
reject;
        };
        neighbor 10.55.55.250 as 65534;
        password "xyz";
        multihop 2;
}
protocol static prefix_aggregation
{
route 192.0.2.0/24 recursive 192.0.2.0;
route 192.0.2.0/24 recursive 192.0.2.1;
route 192.0.2.0/24 recursive 192.0.2.2;
route 192.0.2.0/24 recursive 192.0.2.3;
route 192.0.2.0/24 recursive 192.0.2.4;
route 192.0.2.0/24 recursive 192.0.2.5;
route 192.0.2.0/24 recursive 192.0.2.6;
route 192.0.2.0/24 recursive 192.0.2.7;
route 192.0.2.0/24 recursive 192.0.2.8;
route 192.0.2.0/24 recursive 192.0.2.9;
route 192.0.2.0/24 recursive 192.0.2.10;
route 192.0.2.0/24 recursive 192.0.2.11;
route 192.0.2.0/24 recursive 192.0.2.12;
route 192.0.2.0/24 recursive 192.0.2.13;
route 192.0.2.0/24 recursive 192.0.2.14;
route 192.0.2.0/24 recursive 192.0.2.15;
route 192.0.2.0/24 recursive 192.0.2.16;
route 192.0.2.0/24 recursive 192.0.2.17;
route 192.0.2.0/24 recursive 192.0.2.18;
route 192.0.2.0/24 recursive 192.0.2.19;
route 192.0.2.0/24 recursive 192.0.2.20;
route 192.0.2.0/24 recursive 192.0.2.21;
route 192.0.2.0/24 recursive 192.0.2.22;
route 192.0.2.0/24 recursive 192.0.2.23;
route 192.0.2.0/24 recursive 192.0.2.24;
route 192.0.2.0/24 recursive 192.0.2.25;
route 192.0.2.0/24 recursive 192.0.2.26;
route 192.0.2.0/24 recursive 192.0.2.27;
route 192.0.2.0/24 recursive 192.0.2.28;
route 192.0.2.0/24 recursive 192.0.2.29;
route 192.0.2.0/24 recursive 192.0.2.30;
route 192.0.2.0/24 recursive 192.0.2.31;
route 192.0.2.0/24 recursive 192.0.2.32;
route 192.0.2.0/24 recursive 192.0.2.33;
route 192.0.2.0/24 recursive 192.0.2.34;
route 192.0.2.0/24 recursive 192.0.2.35;
route 192.0.2.0/24 recursive 192.0.2.36;
route 192.0.2.0/24 recursive 192.0.2.37;
route 192.0.2.0/24 recursive 192.0.2.38;
route 192.0.2.0/24 recursive 192.0.2.39;
route 192.0.2.0/24 recursive 192.0.2.40;
route 192.0.2.0/24 recursive 192.0.2.41;
route 192.0.2.0/24 recursive 192.0.2.42;
route 192.0.2.0/24 recursive 192.0.2.43;
route 192.0.2.0/24 recursive 192.0.2.44;
route 192.0.2.0/24 recursive 192.0.2.45;
route 192.0.2.0/24 recursive 192.0.2.46;
route 192.0.2.0/24 recursive 192.0.2.47;
route 192.0.2.0/24 recursive 192.0.2.48;
route 192.0.2.0/24 recursive 192.0.2.49;
route 192.0.2.0/24 recursive 192.0.2.50;
route 192.0.2.0/24 recursive 192.0.2.51;
route 192.0.2.0/24 recursive 192.0.2.52;
route 192.0.2.0/24 recursive 192.0.2.53;
route 192.0.2.0/24 recursive 192.0.2.54;
route 192.0.2.0/24 recursive 192.0.2.55;
route 192.0.2.0/24 recursive 192.0.2.56;
route 192.0.2.0/24 recursive 192.0.2.57;
route 192.0.2.0/24 recursive 192.0.2.58;
route 192.0.2.0/24 recursive 192.0.2.59;
route 192.0.2.0/24 recursive 192.0.2.60;
route 192.0.2.0/24 recursive 192.0.2.61;
route 192.0.2.0/24 recursive 192.0.2.62;
route 192.0.2.0/24 recursive 192.0.2.63;
route 192.0.2.0/24 recursive 192.0.2.64;
route 192.0.2.0/24 recursive 192.0.2.65;
route 192.0.2.0/24 recursive 192.0.2.66;
route 192.0.2.0/24 recursive 192.0.2.67;
route 192.0.2.0/24 recursive 192.0.2.68;
route 192.0.2.0/24 recursive 192.0.2.69;
route 192.0.2.0/24 recursive 192.0.2.70;
route 192.0.2.0/24 recursive 192.0.2.71;
route 192.0.2.0/24 recursive 192.0.2.72;
route 192.0.2.0/24 recursive 192.0.2.73;
route 192.0.2.0/24 recursive 192.0.2.74;
route 192.0.2.0/24 recursive 192.0.2.75;
route 192.0.2.0/24 recursive 192.0.2.76;
route 192.0.2.0/24 recursive 192.0.2.77;
route 192.0.2.0/24 recursive 192.0.2.78;
route 192.0.2.0/24 recursive 192.0.2.79;
route 192.0.2.0/24 recursive 192.0.2.80;
route 192.0.2.0/24 recursive 192.0.2.81;
route 192.0.2.0/24 recursive 192.0.2.82;
route 192.0.2.0/24 recursive 192.0.2.83;
route 192.0.2.0/24 recursive 192.0.2.84;
route 192.0.2.0/24 recursive 192.0.2.85;
route 192.0.2.0/24 recursive 192.0.2.86;
route 192.0.2.0/24 recursive 192.0.2.87;
route 192.0.2.0/24 recursive 192.0.2.88;
route 192.0.2.0/24 recursive 192.0.2.89;
route 192.0.2.0/24 recursive 192.0.2.90;
route 192.0.2.0/24 recursive 192.0.2.91;
route 192.0.2.0/24 recursive 192.0.2.92;
route 192.0.2.0/24 recursive 192.0.2.93;
route 192.0.2.0/24 recursive 192.0.2.94;
route 192.0.2.0/24 recursive 192.0.2.95;
route 192.0.2.0/24 recursive 192.0.2.96;
route 192.0.2.0/24 recursive 192.0.2.97;
route 192.0.2.0/24 recursive 192.0.2.98;
route 192.0.2.0/24 recursive 192.0.2.99;
route 192.0.2.0/24 recursive 192.0.2.100;
route 192.0.2.0/24 recursive 192.0.2.101;
route 192.0.2.0/24 recursive 192.0.2.102;
route 192.0.2.0/24 recursive 192.0.2.103;
route 192.0.2.0/24 recursive 192.0.2.104;
route 192.0.2.0/24 recursive 192.0.2.105;
route 192.0.2.0/24 recursive 192.0.2.106;
route 192.0.2.0/24 recursive 192.0.2.107;
route 192.0.2.0/24 recursive 192.0.2.108;
route 192.0.2.0/24 recursive 192.0.2.109;
route 192.0.2.0/24 recursive 192.0.2.110;
route 192.0.2.0/24 recursive 192.0.2.111;
route 192.0.2.0/24 recursive 192.0.2.112;
route 192.0.2.0/24 recursive 192.0.2.113;
route 192.0.2.0/24 recursive 192.0.2.114;
route 192.0.2.0/24 recursive 192.0.2.115;
route 192.0.2.0/24 recursive 192.0.2.116;
route 192.0.2.0/24 recursive 192.0.2.117;
route 192.0.2.0/24 recursive 192.0.2.118;
route 192.0.2.0/24 recursive 192.0.2.119;
route 192.0.2.0/24 recursive 192.0.2.120;
route 192.0.2.0/24 recursive 192.0.2.121;
route 192.0.2.0/24 recursive 192.0.2.122;
route 192.0.2.0/24 recursive 192.0.2.123;
route 192.0.2.0/24 recursive 192.0.2.124;
route 192.0.2.0/24 recursive 192.0.2.125;
route 192.0.2.0/24 recursive 192.0.2.126;
route 192.0.2.0/24 recursive 192.0.2.127;
route 192.0.2.0/24 recursive 192.0.2.128;
route 192.0.2.0/24 recursive 192.0.2.129;
route 192.0.2.0/24 recursive 192.0.2.130;
route 192.0.2.0/24 recursive 192.0.2.131;
route 192.0.2.0/24 recursive 192.0.2.132;
route 192.0.2.0/24 recursive 192.0.2.133;
route 192.0.2.0/24 recursive 192.0.2.134;
route 192.0.2.0/24 recursive 192.0.2.135;
route 192.0.2.0/24 recursive 192.0.2.136;
route 192.0.2.0/24 recursive 192.0.2.137;
route 192.0.2.0/24 recursive 192.0.2.138;
route 192.0.2.0/24 recursive 192.0.2.139;
route 192.0.2.0/24 recursive 192.0.2.140;
route 192.0.2.0/24 recursive 192.0.2.141;
route 192.0.2.0/24 recursive 192.0.2.142;
route 192.0.2.0/24 recursive 192.0.2.143;
route 192.0.2.0/24 recursive 192.0.2.144;
route 192.0.2.0/24 recursive 192.0.2.145;
route 192.0.2.0/24 recursive 192.0.2.146;
route 192.0.2.0/24 recursive 192.0.2.147;
route 192.0.2.0/24 recursive 192.0.2.148;
route 192.0.2.0/24 recursive 192.0.2.149;
route 192.0.2.0/24 recursive 192.0.2.150;
route 192.0.2.0/24 recursive 192.0.2.151;
route 192.0.2.0/24 recursive 192.0.2.152;
route 192.0.2.0/24 recursive 192.0.2.153;
route 192.0.2.0/24 recursive 192.0.2.154;
route 192.0.2.0/24 recursive 192.0.2.155;
route 192.0.2.0/24 recursive 192.0.2.156;
route 192.0.2.0/24 recursive 192.0.2.157;
route 192.0.2.0/24 recursive 192.0.2.158;
route 192.0.2.0/24 recursive 192.0.2.159;
route 192.0.2.0/24 recursive 192.0.2.160;
route 192.0.2.0/24 recursive 192.0.2.161;
route 192.0.2.0/24 recursive 192.0.2.162;
route 192.0.2.0/24 recursive 192.0.2.163;
route 192.0.2.0/24 recursive 192.0.2.164;
route 192.0.2.0/24 recursive 192.0.2.165;
route 192.0.2.0/24 recursive 192.0.2.166;
route 192.0.2.0/24 recursive 192.0.2.167;
route 192.0.2.0/24 recursive 192.0.2.168;
route 192.0.2.0/24 recursive 192.0.2.169;
route 192.0.2.0/24 recursive 192.0.2.170;
route 192.0.2.0/24 recursive 192.0.2.171;
route 192.0.2.0/24 recursive 192.0.2.172;
route 192.0.2.0/24 recursive 192.0.2.173;
route 192.0.2.0/24 recursive 192.0.2.174;
route 192.0.2.0/24 recursive 192.0.2.175;
route 192.0.2.0/24 recursive 192.0.2.176;
route 192.0.2.0/24 recursive 192.0.2.177;
route 192.0.2.0/24 recursive 192.0.2.178;
route 192.0.2.0/24 recursive 192.0.2.179;
route 192.0.2.0/24 recursive 192.0.2.180;
route 192.0.2.0/24 recursive 192.0.2.181;
route 192.0.2.0/24 recursive 192.0.2.182;
route 192.0.2.0/24 recursive 192.0.2.183;
route 192.0.2.0/24 recursive 192.0.2.184;
route 192.0.2.0/24 recursive 192.0.2.185;
route 192.0.2.0/24 recursive 192.0.2.186;
route 192.0.2.0/24 recursive 192.0.2.187;
route 192.0.2.0/24 recursive 192.0.2.188;
route 192.0.2.0/24 recursive 192.0.2.189;
route 192.0.2.0/24 recursive 192.0.2.190;
route 192.0.2.0/24 recursive 192.0.2.191;
route 192.0.2.0/24 recursive 192.0.2.192;
route 192.0.2.0/24 recursive 192.0.2.193;
route 192.0.2.0/24 recursive 192.0.2.194;
route 192.0.2.0/24 recursive 192.0.2.195;
route 192.0.2.0/24 recursive 192.0.2.196;
route 192.0.2.0/24 recursive 192.0.2.197;
route 192.0.2.0/24 recursive 192.0.2.198;
route 192.0.2.0/24 recursive 192.0.2.199;
route 192.0.2.0/24 recursive 192.0.2.200;
route 192.0.2.0/24 recursive 192.0.2.201;
route 192.0.2.0/24 recursive 192.0.2.202;
route 192.0.2.0/24 recursive 192.0.2.203;
route 192.0.2.0/24 recursive 192.0.2.204;
route 192.0.2.0/24 recursive 192.0.2.205;
route 192.0.2.0/24 recursive 192.0.2.206;
route 192.0.2.0/24 recursive 192.0.2.207;
route 192.0.2.0/24 recursive 192.0.2.208;
route 192.0.2.0/24 recursive 192.0.2.209; # <-- this one should
be reachable
route 192.0.2.0/24 recursive 192.0.2.210;
route 192.0.2.0/24 recursive 192.0.2.211;
route 192.0.2.0/24 recursive 192.0.2.212;
route 192.0.2.0/24 recursive 192.0.2.213;
route 192.0.2.0/24 recursive 192.0.2.214;
route 192.0.2.0/24 recursive 192.0.2.215;
route 192.0.2.0/24 recursive 192.0.2.216;
route 192.0.2.0/24 recursive 192.0.2.217;
route 192.0.2.0/24 recursive 192.0.2.218;
route 192.0.2.0/24 recursive 192.0.2.219;
route 192.0.2.0/24 recursive 192.0.2.220;
route 192.0.2.0/24 recursive 192.0.2.221;
route 192.0.2.0/24 recursive 192.0.2.222;
route 192.0.2.0/24 recursive 192.0.2.223;
route 192.0.2.0/24 recursive 192.0.2.224;
route 192.0.2.0/24 recursive 192.0.2.225;
route 192.0.2.0/24 recursive 192.0.2.226;
route 192.0.2.0/24 recursive 192.0.2.227;
route 192.0.2.0/24 recursive 192.0.2.228;
route 192.0.2.0/24 recursive 192.0.2.229;
route 192.0.2.0/24 recursive 192.0.2.230;
route 192.0.2.0/24 recursive 192.0.2.231;
route 192.0.2.0/24 recursive 192.0.2.232;
route 192.0.2.0/24 recursive 192.0.2.233;
route 192.0.2.0/24 recursive 192.0.2.234;
route 192.0.2.0/24 recursive 192.0.2.235;
route 192.0.2.0/24 recursive 192.0.2.236;
route 192.0.2.0/24 recursive 192.0.2.237;
route 192.0.2.0/24 recursive 192.0.2.238;
route 192.0.2.0/24 recursive 192.0.2.239;
route 192.0.2.0/24 recursive 192.0.2.240;
route 192.0.2.0/24 recursive 192.0.2.241;
route 192.0.2.0/24 recursive 192.0.2.242;
route 192.0.2.0/24 recursive 192.0.2.243;
route 192.0.2.0/24 recursive 192.0.2.244;
route 192.0.2.0/24 recursive 192.0.2.245;
route 192.0.2.0/24 recursive 192.0.2.246;
route 192.0.2.0/24 recursive 192.0.2.247;
route 192.0.2.0/24 recursive 192.0.2.248;
route 192.0.2.0/24 recursive 192.0.2.249;
route 192.0.2.0/24 recursive 192.0.2.250;
route 192.0.2.0/24 recursive 192.0.2.251;
route 192.0.2.0/24 recursive 192.0.2.252;
route 192.0.2.0/24 recursive 192.0.2.253;
route 192.0.2.0/24 recursive 192.0.2.254;
route 192.0.2.0/24 recursive 192.0.2.255;
}
I then set up a dummy device (and add it to OSPF so that it lands
in birds routing table):
# ip link add dum0 type dummy
# ip addr add 192.0.2.209/28 dev dum0
# ip link set dev dum0 up
# ping -c1 192.0.2.209
PING 192.0.2.209 (192.0.2.209) 56(84) bytes of data.
64 bytes from 192.0.2.209: icmp_seq=1 ttl=64 time=0.022 ms
--- 192.0.2.209 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.022/0.022/0.022/0.000 ms
But then:
# birdc show route | grep '^192\.0\.2\.'
192.0.2.208/28     dev dum0 [ospf 04:57:58] * I (150/10)
[14x.xxx.223.80]
192.0.2.0/24       unreachable [prefix_aggregation 04:57:57] *
(200)
#
The 192.0.2.208/28 shows up in the routing table, so it IS
reachable and hence at least one of the 192.0.2.0/24 route entries much
be reachable. Yet, 192.0.2.0/24 shows up as unreachable.
Thanks,
Luke
...
Gesendet: Sonntag, 14. Januar 2024 um 00:24 Uhr
Von: "Alexander Zubkov" <green@qrator.net>
An: "Lukas Haase" <lukashaase@gmx.at>
Cc: bird-users@network.cz
Betreff: Re: Exporting a larger prefix if a smaller prefix is
being exported
Hi Lukas,
On Sun, Jan 14, 2024 at 6:23 AM Lukas Haase <lukashaase@gmx.at>
wrote:
>
> Hi Alex,
>
> > Gesendet: Samstag, 13. Januar 2024 um 06:31 Uhr
> > Von: "Alexander Zubkov" <green@qrator.net>
> > An: "Lukas Haase" <lukashaase@gmx.at>
> > Cc: bird-users@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;
> }
Yes, something like that. But it needs to be "recursive" not
"via".
And your routes need not to be recursive already (for example
iBGP
makes such routes by default), because double recursion won't
work.
>
> If so, how do I avoid that 192.0.2.0/24 will be exported five
times?
Yes, you'll have multiple similar routes. But if you do not have
add-path on your BGP session, only one of them will be exported.
Also
in the recent version there is some aggregation support of
same-net
prefixes, it can be helpful here too.
> And how do I set up an export filter on "next-hop is not
reachable"?
You need to check the corresponding attribute of the prefix. See
the
example later.
>
> By the way, the sub-prefixes, would I just export via a filter
like this?
Yes, this filter seems to cover the sub-prefixes. But not sure
what is
the nature of this question.
>
> export filter {
>     if (net ~ [192.0.2.0/24{25,32}
<http://192.0.2.0/24%7B25,32%7D>]) then {
>         accept;
>     }
>     reject;
> }
>
Here is a config example I can imagine, not tested it at all.
Something like that works for us. But I would test it first that
it
correctly adds/reverts the "aggregated" prefix. It adds static
prefixes in a separate table, but it can also be done in the main
table and reachability tested in bgp export filter for example.
ipv4 table aggr;
protocol static prefix_aggregation
{
  ipv4 { table aggr4; };
  route 192.0.2.0/24 recursive 192.0.2.209;
  ...
}
protocol pipe pipe_aggr
{
  table master4;
  peer table aggr;
  export filter {
    if (net ~ [192.0.2.0/24{25,32}
<http://192.0.2.0/24%7B25,32%7D>]) then {
      accept;
    }
    reject;
  };
  import filter {
    if dest = RTD_UNREACHABLE then reject;
    if (net = 192.0.2.0/24) 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@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
> > >
> > >
> >

Alexander Zubkov

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