How to change route gw on export to BGP neighbor?

[Alexander Zubkov]

Hi,

There is no reason to set gw attribute in the export filter, because
this attribute is not exported over bgp. bgp_next_hop is what you
want. You should also consider the "next hop self" option.

On Mon, Nov 2, 2020 at 5:53 PM ico <ico at petrzalka.net> wrote:
>
> Hello,
>
> I'm trying to to export some routes from our internal OSPF to BGP , both
> running in the same bird instance. I am able to filter those routes via
> BGP just fine (some tables connected via pipe). The problem is that all
> exported routes have private gw addresses (10.x.x.x). I am unable to
> configure bird to change gw of routes before then get sent to neighbors.
> Neighbors see original gateways from 10.0.0.0/8 range which is
> invalid/unreachable from outside of course.
>
> Here is also some simplyfied example, not showing OSPF setup etc. In
> this example I also don't show the pipes between tables. All of those
> doesn't seem to be important in this case.
>
> ########
> router id 1.1.1.2;
>
> protocol kernel krnl4 {
>      ipv4 {
>          export all;
>          import all; # route 2.2.2.0/24 gets into bird here
>      };
>      learn;
> }
> protocol device {
> }
>
> protocol bgp {
>      local 1.1.1.2 as 65000;
>      neighbor 1.1.1.1 as 65000;
>      strict bind;
>      ipv4 {
>          import none;
>          export filter {
>              # here I want to export routes with changed gateways
>              bgp_next_hop = 1.1.1.5; # sets next hop but not gw
>              gw = 1.1.1.5; # <ERR> filters, line XY: Invalid gw address
>              print "export bgp ", net, " gw ", gw, " bnh ", bgp_next_hop;
>              # <INFO> export bgp 2.2.2.0/24 gw 1.1.1.5 bnh 1.1.1.5
>              accept;
>          };
>      };
> }
> ########
>
> With similar setup in a lab I was able to change those gw attributes on
> RECEIVING side in import filter to any arbitrary value I wanted. In real
> life I don't manage those BGP routers so I need to make sure to send
> correct routes.
>
> The example config prints changed gw but BGP peer sees the original gw.
> In some tests I got error "Invalid gw address" but now I don't have that
> config. Here it just ignores changed gw.
>
> Is there a configuration to achieve this?
>
> Thanks
>
> ico