changing bgp_med attributes cause route been filited

[Brandon Zhi]

Hi Ondrej,

Thanks for the advice, I have fixed with. I always use Python, so I guess
other C like Stript needs to do that.

            # 如果 MED 值已经存在，则将其以原值为基础增加
            if defined (bgp_med) then {
                bgp_med = bgp_med + bgp_med_ping + bgp_med_ping;
            }
            # 否则，设置 MED 值为新值
            else {
                bgp_med = bgp_med_ping + bgp_med_ping;
            }

             return true;

AIGP is not intended to be transitive between completely distinct
> autonomous systems (only across internal AS boundaries)

AIGP is always compared in paths that have the attribute, regardless of
> whether they come from different neighbor AS or not
> AIGP is more important than MED in the BGP decision process (see the
> section titled BGP decision process)
> AIGP is automatically incremented every time there is a BGP next-hop
> change so that it can track the end-to-end IGP cost. All arithmetic
> operations on MED attributes must be done manually (for example, using
> route policies)



Looks like local_pref > bgp_path > aigp > bgp_med? And aigp is added when
passing each ibgp session Automatically.?

Best regard,
*Brandon Zhi*
HUIZE LTD
www.huize.asia  <https://huize.asia/>| www.ixp.su | Twitter

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On Thu, 27 Jul 2023 at 19:32, Douglas Fischer <fischerdouglas at gmail.com>
wrote:

> This reminds me of a meme...
> https://img-comment-fun.9cache.com/media/ay9BKnV/aWlGMBgm_700w_0.jpg
>
> Em qua., 26 de jul. de 2023 às 20:54, Ondrej Zajicek <
> santiago at crfreenet.org> escreveu:
>
>> On Thu, Jul 20, 2023 at 08:25:24PM +0800, Brandon Zhi wrote:
>> > Hi all,
>> >
>> > I was doing research about selecting the best route on multiple nodes
>> based
>> > on bgp_med attributes.
>> >
>> >
>> > My idea is to add bgp_med value between each node based on ping, and
>> then
>> > bird will select the route which has the lowest bgp_med value (which is
>> the
>> > total ping delay)
>> >
>> > But some routes will be filled, which i don't want it.
>> >
>> > I found that the code " bgp_med = bgp_med + bgp_med_ping +
>> bgp_med_ping;"
>> > will cause some routes to be filtered.
>> >
>> > Can someone tell me why? I only want to modify bgp_med and don't want my
>> > routes to be filtered.
>>
>> Hi
>>
>> Perhaps there are some error in logs about filter failures? I guess that
>> when bgp_med is not defined, then expression 'bgp_med + X' fails.
>>
>> You could try something like:
>>
>>   if (defined(bgp_med))
>>     bgp_med = bgp_med + X;
>>   else
>>     bgp_med = X;
>>
>>
>> And perhaps even better than use bgp_med for this is to use bgp_aigp
>> attribute and associated BGP options (aigp, cost).
>>
>> --
>> Elen sila lumenn' omentielvo
>>
>> Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
>> OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
>> "To err is human -- to blame it on a computer is even more so."
>>
>
>
> --
> Douglas Fernando Fischer
> Engº de Controle e Automação
>
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