BIRD: BGP filter example



Coming from Junos, I found that manipulating BGP path attributes in BIRD is both straightforward and powerful. I wanted to share an example filter that manipulates the BGP PATH attributes that are changed most often.

I used BIRD as a route-reflector for two MX routers to test the filter. The filter is setup sort of like it can be found on a lot of other platforms. A simple list that results in a terminating action as soon as there is a match.

The filter:

[root@bird ~]# more /etc/bird.filters
filter rr_export {
        # term 1 | set community to 1:100
        if (net = 192.168.1.0/24) then
            {
                bgp_community.add ((1,100));
                accept;
            }
        # term 2 | set community to 1:1000
        if (net = 192.168.2.0/24) then
            {
                bgp_community.add ((1,1000));
                accept;
            }
        # term 3 | set local preference to 175
        if (net = 192.168.3.0/24) then
            {
                bgp_local_pref=175;
                accept;
            }
        # term 4 | prepend an AS
        if (net = 192.168.4.0/24) then
            {
                bgp_path.prepend(65000);
                accept;
            }
        # term 5 | alter the next-hop to whatever (useful for RTBH)
        # make sure BIRD can resolve next-hop, even when used as export filter
        if (net = 192.168.5.0/24) then
            {
                bgp_next_hop = 34.0.1.1;        
                accept;
            }
        # term 6 | alter the MED
        if (net = 192.168.6.0/24) then
            {
                bgp_med  = 175;
                accept;
            }
        # term 7 | match on subnet AND community,
        # then changes two PATH attributes and strip communities
        if (net = 192.168.7.0/24 && (541,541) ~ bgp_community) then
            {
                bgp_community.empty;
                bgp_med  = 241;
                bgp_local_pref=175;
                accept;
            }
        # term 8 | match on subnet or MED
        if (net = 192.168.8.0/24 || bgp_med = 321) then
            {
                bgp_med  = 8;
                bgp_local_pref=8;
                accept;
            }
        # term 9 | match prefix-list style, 
        # example is for any prefix with a mask ranging from 20 - 24
        # Junos: route-filter 0.0.0.0/0 prefix-length-range /20-/24;
        if ( net ~ [ 0.0.0.0/0{20,24} ] ) then
            {
                bgp_med  = 88;
                bgp_local_pref=88;
                accept;
            }
        # end by accepting all other routes
        else accept;
    }

To apply the filter, reference it with the following configuration command:

template bgp RR {
..
        export filter rr_export;                                 
..
}

Worked for me and I wanted to keep it as an example to copy paste from in the future.

Chapter 5 in the BIRD User's Guide offers some very clear insights into all the things possible with filtering and BIRD.

15-6-2015