Hi Alexander
> Gesendet: Sonntag, 14. Januar 2024 um 23:03 Uhr
> Von: "Alexander Zubkov" <gr...@qrator.net>
> An: "Lukas Haase" <lukasha...@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 <lukasha...@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" <gr...@qrator.net>
> > > An: "Lukas Haase" <lukasha...@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 <lukasha...@gmx.at> wrote:
> > > >
> > > > Hi Alex,
> > > >
> > > > > Gesendet: Samstag, 13. Januar 2024 um 06:31 Uhr
> > > > > Von: "Alexander Zubkov" <gr...@qrator.net>
> > > > > An: "Lukas Haase" <lukasha...@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}]) 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@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
> > > > > >
> > > > > >
> > > > >
> > >
>
