How to change route gw on export to BGP neighbor?

[Alexander Zubkov]

Hi,

You need to show a full routing table on the receiver. Probably you do
not have your direct route for 1.1.1.0/24 (or what you have) in the
bird's table, so it considers gateway 1.1.1.5 to be unreachable. I
usually add those routes with protocol direct. Pay attention also that
your gateway will be tried to be calculated recursively because it is
a default for an iBGP session. So if you had 0.0.0.0 via 2.2.2.2 in
your table, you would receive gw = 2.2.2.2 with bgp_next_hop =
1.1.1.5.
Heh, I just talked about that at the RIPE meeting several days ago:
https://ripe81.ripe.net/archives/video/479/ (direct/recursive stuff
begins at 10:25). :)

On Wed, Nov 4, 2020 at 2:15 PM ico <ico at petrzalka.net> wrote:
>
> Hi,
>
> Thank you for reply. Unfortunately that seems not to solve my problem.
> Config of my bird instance is in previous email. The "receiving peer" I
> set in test lab running bird is configured as follows:
>
> ########
> router id 1.1.1.1;
>
> protocol bgp {
>      local 1.1.1.1 as 65000;
>      neighbor 1.1.1.2 as 65000;
>      strict bind;
>      ipv4 {
>          import filter {
>              print "import bgp ", net, " gw ", gw, " bnh ", bgp_next_hop;
>              #gw = 1.1.1.5;
>              accept;
>          };
>          export none;
>      };
> }
> ########
>
> This should simply receive a route 2.2.2.0/24 with whatever gw attribute
> and with bgp_next_hop set to 1.1.1.5. This is of course received, but
> the gw is not set to 1.1.1.5 on receiver. The print statement produces this:
>
> <INFO> import bgp 2.2.2.0/24 gw :: bnh 1.1.1.5
>
> Receiver's console:
>
> bird> show route all
> Table master4:
> 2.2.2.0/24           unreachable [bgp1 13:54:09.354 from 1.1.1.2] *
> (100) [i]
>          Type: BGP univ
>          BGP.origin: IGP
>          BGP.as_path:
>          BGP.next_hop: 1.1.1.5
>          BGP.local_pref: 100
>
> As you can see the next hop is 1.1.1.5 but still it is unreachable. I
> can set it on receiver side (with commented out "gw = 1.1.1.5;"), but
> that will require a change on receiver, which I am unable to do in my
> situation. Is there a workaround for this?
>
> Thanks
>
> ico
>
> On 2. 11. 2020 19:26, Alexander Zubkov wrote:
> > 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