Exporting a larger prefix if a smaller prefix is being exported
Lukas Haase
lukashaase at gmx.at
Mon Jan 15 08:59:39 CET 2024
Hi Alexander
> Gesendet: Sonntag, 14. Januar 2024 um 23:03 Uhr
> Von: "Alexander Zubkov" <green at qrator.net>
> An: "Lukas Haase" <lukashaase at gmx.at>
> Cc: bird-users at 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 at 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}]) 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 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 Lukas,
> > >
> > > On Sun, Jan 14, 2024 at 6:23 AM Lukas Haase <lukashaase at gmx.at> wrote:
> > > >
> > > > 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;
> > > > }
> > >
> > > 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}]) 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}]) 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 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
> > > > > >
> > > > > >
> > > > >
> > >
>
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