[isp-bgp] Re: MEDs from different ASes

["Tulip Rasputin" <tulip_rasputin@...>]

Hi Jim,

Thanks for the response. I think i wasnt very clear with my question. Too
little coffee .. eh ?!?

I am aware of how we can configure the boxes so that the MEDs are always
compared, even across the ASes. What i wanted to know was, *why* would
somebody ever want to do that?

Regards,
Rasputin

----- Original Message ----- 
From: "James" <haesu@...>
To: <isp-bgp@isp-bgp.com>
Sent: Thursday, May 20, 2004 7:00 PM
Subject: [isp-bgp] Re: MEDs from different ASes


On Thu, May 20, 2004 at 12:08:37PM +0530, Tulip Rasputin wrote:
> Hello,
>
> I am able to comprehend the concept of comparing MEDs for the routes that
we
> recieve from the same AS.

yes, that's what a MED is for. it's designed to let you 'view' the
"topology"
of your peer's setup when peering at multi locations. this is the default
behaviour, also as deterministic-med


> What confuses me is when, would we really want to
> compare MEDs across different ASes?

conf t
router bgp ASN
bgp always-compare-med


> MEDs can mean different things to
> different operators, one could be reflecting the internal IGP costs in the
> MED, the other could be doing something totally different. How and in what
> scenario, will we want to compare MEDs across different ASes?
>
> Can anybody throw any light on this? Has anybody ever done that? And if
yes,
> then when?

yes i have. honestly, i don't like doing it that way.

>
> One reason which i can think of at the top of my head is to avoid RFC 3345
> oscillations. We may use always-compare-MED to prevent those. But then
there
> are many topological solutions that can help prevent those oscillations.

always-compare-med compares MED between varying ASes. remember that if a
path
has smaller as path, it will still win (med comes after as path check)

-J


>
> Thanks,
> Rasputin
>
>
>
>




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-- 
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Inc.
Technical Lead                        Network Design, Consulting, IT
Outsourcing
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Services
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