BGP route back

[Roman Romanyak]

I'd like to do a route manipulation on the route server and send a route
back also to router1 with a local pref via ISP1.

On Sat, Jan 19, 2019 at 2:43 AM Alexander Zubkov <green at qrator.net> wrote:

> Hi,
>
> Thank you for a good description. But what's your problem there now?
> You receive x.y.z.0/24 from Router1 and send it to Router2. What do
> you want to achive and are missing?
>
> On Fri, Jan 18, 2019 at 9:13 PM Roman Romanyak <roman.romanyak at gmail.com>
> wrote:
> >
> > 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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