bird route selection problem

[Daniel Rimal]

On Fri, Jan 28, 2011 at 10:07 AM, Ondrej Zajicek <santiago at crfreenet.org> wrote:
> On Thu, Jan 27, 2011 at 05:50:42PM +0100, Daniel Rimal wrote:
>> Hello,
>>
>> i have little weird problem with route selection in bird. A have four
>> routers in
>> my AS, routers called R1 and R2 is running guagga, routers called R3 and R4
>> is running bird 1.2.5. I have configured full mesh between all of them.
>>
>> IP 81.xxx.yyy.189 and 81.xxx.yyy.190 is ebgp peers. Both,
>> R3 and R4 running bird have established peer with both ebgp routers.
>>
>> IP 109.xxx.yyy.11 and 109.xxx.yyy.12 is IP on R3 respectively R4
>> (ibgp interconnection)
>>
>> After receive this withdraw from ebgp peer 81.xxx.yyy.189 by R3 for example:
>> 14:19:23.930683 81.xxx.yyy.189  81.x.yyy.185    BGP     UPDATE Message
>> Withdrawn prefix: 187.120.96.0 (187.120.96.0)
>>
>> bird do this: - switch route between ebgp and ibgp peer very fast -
>> 288 times (in one second) in this case:
>> 14:19:25 <TRACE> kernel1 < replaced 187.120.96.0/23 via 81.xxx.yyy.190
>> on bond0.402
>> 14:19:25 <TRACE> kernel1 < replaced 187.120.96.0/23 via 109.xxx.yyy.12
>> on bond0.403
>> 14:19:25 <TRACE> kernel1 < replaced 187.120.96.0/23 via 81.xxx.yyy.190
>> on bond0.402
>
> This is really strange - if R3 has a route from 81.xxx.yyy.190, that
> route should be preferred. One possible explanation is that ebgp peer
> 81.xxx.yyy.190 also sent that sequence of updates and withdraws to R3
> (causing him to switch to ibgp-route when ebgp-route was withdrawn and
> swich back when it reappears). Are you sure that it is not the case?
>
> The switching sequence was on both R3 and R4?
>
> Could you also send me an output of 'show route 187.120.96.0/23 all'
> on R3 and R4?

There is route detail:

187.120.96.0/23    via 81.xxx.yyy.190 on bond0.402 [casa190 04:45] *
(100) [AS28202i]
        Type: BGP unicast univ
        BGP.origin: IGP
        BGP.as_path: 15685 29208 3549 4230 28202
        BGP.next_hop: 81.xxx.yyy.190
        BGP.local_pref: 700
        BGP.community:
                   via 109.xxx.yyy.10 on bond0.403 [r2 09:08] (100) [AS28202i]
        Type: BGP unicast univ
        BGP.origin: IGP
        BGP.as_path: 29208 3549 4230 28202
        BGP.next_hop: 109.xxx.yyy.10
        BGP.local_pref: 500
        BGP.community: (2,100) (2,3549) (3549,4359) (3549,30840)
(4230,21) (4230,30) (4230,31) (4230,131) (29208,3549)
                   via 81.xxx.yyy.189 on bond0.402 [casa189 04:45]
(100) [AS28202i]
        Type: BGP unicast univ
        BGP.origin: IGP
        BGP.as_path: 15685 29208 3549 4230 28202
        BGP.next_hop: 81.xxx.yyy.189
        BGP.local_pref: 700
        BGP.community:

But route list is probably affected by some experiments which i did on
routers last night. I'm sure that ebgp routers send only one
update/withdraw, i checked by tcpdump too. Switching sequence was on
R3 and R4 too.

But i found the problem is probably caused by local_pref settings. I
did some experiments and found bird weird behaviour when i have
identical bgp_local_pref value in import filter on both ibgp peers R3
& R4, like that:

filter bgp_in_sitel
    {
            bgp_local_pref=500;
            accept;
    }

protocol bgp r4 {
#       disabled;
        description "propoj s R4";
        debug {events, states};
        local as 35236;
        neighbor 109.xxx.yyy.12 as 35236;
        hold time 90;
        startup hold time 90;
        connect retry time 30;
        keepalive time 30;
        start delay time 5;
        error wait time 60, 300;
        error forget time 300;
#       disable after error;
        next hop self;
        path metric 1;
        default bgp_med 0;
        default bgp_local_pref 500;
        import filter bgp_in_sitel;
        export all;

When i completely disabled import filter on both ibgp peers (set
import all) and "default_bgp_local_pref" was identical on R3 & R4,
bird behaviour was normal.

When i enabled import filter on both sides, set different
"bgp_local_pref" but "default bgp_local_pref" was identical, bird
behaviour was still bad.

When i enabled import filter on both sides, set different
"bgp_local_pref" and set different "default bgp_local_pref" , bird
behaviour was normal.

Dan