[isp-bgp] Re: MEDs from different ASes

[James <haesu@...>]

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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-- 
James Jun                                            TowardEX Technologies, Inc.
Technical Lead                        Network Design, Consulting, IT Outsourcing
james@...                  Boston-based Colocation & Bandwidth Services
cell: 1(978)-394-2867           web: http://www.towardex.com , noc: www.twdx.net






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