bgp_path.first

[Alarig Le Lay]

On lun. 21 août 20:29:25 2017, nixx wrote:
> Hello.
> 
> i'm trying to configure bird for working with uplinks and downlinks with
> this example:
> https://gitlab.labs.nic.cz/labs/bird/wikis/BGP_filtering
> 
> i'm using truncated function:
> 
> function rt_export()
> {
>   if proto = "static_bgp" then return true;
>   if source != RTS_BGP then return false;
>   return bgp_path.first ~ [ downlink1AS, downlink2AS ];
> }
> 
> But this incorrectly works - bird trying to export my full-view to uplink.
> 
> Then, when i changed to
> 
> if ( bgp_path.len = 2 ) && ( bgp_path.first = myAS ) then return bgp_path ~
> [ downlink1AS, downlink2AS ];
> 
> ...it works.
> 
> What i'm (or my bird) doing wrong? Or maybe this example is not correct?

Hi,

When you export a route with BGP, the first ASN is yours, unless you act
as a route-reflector or a route-server. But the first will never be any
of your downstreams.

On the above example, the first range originate from the organisation
ASN and the next are transit customers :
% birdc6 show route all export bgp_quantic
BIRD 1.6.3 ready.
2a00:5884::/32     unreachable [static_grifon_allocations 2017-07-29] * (200)
	Type: static unicast univ
	BGP.origin: IGP
	BGP.as_path: 204092
	BGP.next_hop: 2a06:e040:3501:101:2::2
	BGP.local_pref: 100
2a06:e881:113::/48 via 2a00:5884:0:101::10 on em1.101 [bgp_guizmo34 2017-07-29] * (100) [AS205816i]
	Type: BGP unicast univ
	BGP.origin: IGP
	BGP.as_path: 204092 205816
	BGP.next_hop: 2a06:e040:3501:101:2::2
	BGP.med: 0
	BGP.local_pref: 200
2001:678:3cc::/48  via 2a00:5884::25 on em1 [bgp_petrus 2017-07-29] * (100) [AS206155i]
	Type: BGP unicast univ
	BGP.origin: IGP
	BGP.as_path: 204092 206155
	BGP.next_hop: 2a06:e040:3501:101:2::2
	BGP.local_pref: 200
	BGP.large_community: (206155, 200, 0)

-- 
alarig
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