BGP route back

[Roman Romanyak]

Alexander, thank you for your feedback.

Here is a diagram:

[ ISP1 ]      [ ISP2 ]                            [ ISP3 ]        [ ISP4 ]
     |                 |
|                   |
     |                 |
|                   |
[       Router 1        ]                           [       Router 2
]
               |                                                         |
               |                                                         |
  [                        Layer 2 Switch                               ]
                                      |
                                      |
                       [ Bird Route Server ]


Router 1 and 2 do not peer directly. There is OSPF running, so all nodes
learn a route to other nodes connected interfaces.
Both routers peer to route-server and are route reflection  clients.

There is a route x.y.z.0/24 coming from from each ISP. Router 1 chooses
ISP2 (because of as-path), what path Router 2 is choosing doesn't matter.

I need to force traffic to x.y.z.0/24 via ISP1.
In order to do that, I'm matching the route x.y.z.0/24 in the import filter
on the route-server (maching a route itself  and also ISP1 ASN in the path)
and setting the local preference to 150.
Then this route is re-advertised to Router 2, which will select it as best,
because of iBGP and high local-preference. The route is not announced back
to Router 1, because it came from there.

An alternative way, to do that is to mach a route on Router 1, increase a
local-pref there and that route will be leared by router 2 through the
route-server. But I'd like to do that on route-server to use a centralized
place for these kinds of things.

Here is a show route output from a route server:


bird> show route for x.y.z.0/24 all
x.y.z.0/24   via 10.1.1.65 on em2 [edge_r1 18:25:09] * (100/11) [AS123i]
    Type: BGP unicast univ
    BGP.origin: IGP
    BGP.as_path: 111 888 333 999
    BGP.next_hop: 1.1.1.1
    BGP.med: 0
    BGP.local_pref: 150
                       via 10.1.1.65 on em2 [edge_r1 2019-01-17] (100/11)
[AS123i]
    Type: BGP unicast univ
    BGP.origin: IGP
    BGP.as_path: 222 777 999
    BGP.next_hop: 2.2.2.2
    BGP.med: 0
    BGP.local_pref: 100
                       via 10.1.1.66 on em2 [edge_r2 2019-01-16] (100/11)
[AS123i]
    Type: BGP unicast univ
    BGP.origin: IGP
    BGP.as_path: 333 666 999
    BGP.next_hop: 3.3.3.3
    BGP.med: 0
    BGP.local_pref: 100
                       via 10.1.1.67 on em2 [edge_r2 2019-01-17] (100/11)
[AS123i]
    Type: BGP unicast univ
    BGP.origin: IGP
    BGP.as_path: 444 555 999
    BGP.next_hop: 4.4.4.4
    BGP.med: 0
    BGP.local_pref: 100

bird>






On Fri, Jan 18, 2019 at 2:17 PM Alexander Zubkov <green at qrator.net> wrote:

> Hi,
>
> I do not completely understand your setup. You better show route
> tables from all the routers and what is announced in which direction.
>
> > So I get the desired effect on the second router, it will learn and
> install a route with high local pref. The first router where the desired
> transit link is connected by default selects another transit link because
> of the shorter as-path.
>
> How this could happen? If you got this route on the route reflector
> from the first router, than it should have this route in the required
> direction. If it had this route to another transit link, it would
> announce this to the route reflector. There is no reason to propagate
> the routes back to the peer, because they are already there.I think
> you can do some tricks, but first we need to understand what do you
> want to achieve.
>
> On Fri, Jan 18, 2019 at 7:23 PM Roman Romanyak <roman.romanyak at gmail.com>
> wrote:
> >
> > Hello Bird users,
> >
> > Does anyone know if there is a way to announce a BGP route back to the
> router it was received from.
> > I need to do this for the following scenario.
> > Lets say there are two routers with 2 transit links on each with full
> view tables and Bird server as a route-reflector. There is a need to force
> traffic to a specific destination via one of the ISPs, so I match the route
> in the import filter  and set a local pref on it. But that will only make
> the route server announce the route with a high local pref to a second
> router, where the desired transit isn't directly connected. So I get the
> desired effect on the second router, it will learn and install a route with
> high local pref. The first router where the desired transit link is
> connected by default selects another transit link because of the shorter
> as-path.
> >
> > Here is the import filter snippet (x.y.z.0/24 is a dest route, as-path
> 1234 is a directly connected ISP on router-1:
> >
> >         if source = RTS_BGP && net = x.y.z.0/24 && bgp_path.first = 1234
> then {
> >             bgp_local_pref = 150;
> >             accept;
> >         }
> >
> > I think that bird doesn't do that because the protocol matches on the
> peer and on the route.
> >
> >
> > Thanks!
>
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