Re: MPLS VPN Design and BGP SoO

Hi Karim,

That supports what I was thinking -- the only time you would ever need to
use BGP SoO is when you have multiple sites using the same BGP ASN and some
of them are multihomed. In that case, you would have to do either
'as-override' or 'allowas-in', but you wouldn't want to have routes enter
via one PE and then get re-advertised via the other PE. So, we use SoO to
prevent these looping updates from being re-accepted into the
originally-advertising site.

Is that about right?

Keller Giacomarro
keller.g_at_gmail.com

On Mon, Oct 8, 2012 at 2:22 AM, Karim Jamali <karim.jamali_at_gmail.com> wrote:

> Hi Keller,
>
> The only scenario I have came across is suppose you have two sites one is
> dual homed to two PE routers and another site connected to a third PE
> within an MPLS VPN using the same AS. When you use as-override for instance
> routes can be propagated between sites. Thus we have a problem that a route
> originated within site 1 can go to PE1 to PE2 which will do the as-override
> and send it CE2 within the same site. Thus the BGP SOO or any community can
> be used to filter this behavior.
>
> Thanks
> On Oct 8, 2012 10:11 AM, "Gaurav Thukral" <pearlgaurav_at_gmail.com> wrote:
>
>> Hi Keller
>>
>> It's my fault. Let me go through the requirement.
>>
>> Thanks
>> Gaurav
>>
>> On Mon, Oct 8, 2012 at 12:24 PM, Keller Giacomarro <keller.g_at_gmail.com
>> >wrote:
>>
>> > Hi Gaurav,
>> >
>> > I have read through that doc, and it addresses EIGRP SoO only. My
>> > questions are specifically about BGP SoO.
>> >
>> > Keller Giacomarro
>> > keller.g_at_gmail.com
>> >
>> >
>> >
>> > On Mon, Oct 8, 2012 at 1:48 AM, Gaurav Thukral <pearlgaurav_at_gmail.com
>> >wrote:
>> >
>> >> Hi
>> >>
>> >> Kindly go through the below doc.
>> >>
>> >>
>> >>
>> http://blog.ine.com/wp-content/uploads/2010/04/understanding-eigrp-soo-bgp-cost-community.pdf
>> >>
>> >> Thanks
>> >> Gaurav
>> >>
>> >> On Mon, Oct 8, 2012 at 9:48 AM, Keller Giacomarro <keller.g_at_gmail.com
>> >wrote:
>> >>
>> >>> I am having issues understanding when to use SoO where. These are the
>> >>> scenarios I can think of where we'd be using BGP as the IGP and would
>> >>> consider the use of some of these more advanced BGP multi-homing
>> >>> features...
>> >>>
>> >>> Two sites, different ASNs, no backdoor link
>> >>> - no special config needed
>> >>>
>> >>> Two sites, same ASN, no backdoor link
>> >>> - use either allowas-in or as-override to allow the sites to accept
>> >>> routes
>> >>> from each other
>> >>> - if any of the site are multi-homed, use SoO to keep the routes from
>> >>> circling back into the same site via the other PE
>> >>>
>> >>> Two sites, different ASNs, with backdoor (backup-only) link
>> >>> - peer the backdoor CE routers via eBGP and prepend to prefer the MPLS
>> >>> link
>> >>>
>> >>> Two sites, same ASN, with backdoor (backup-only) link
>> >>> - ???
>> >>>
>> >>> One site, one ASN, multiple MPLS exit points, with intra-site active
>> >>> (non-backup) links
>> >>> - no special config needed, but MPLS cannot be used if the internal
>> site
>> >>> becomes segmented (due to AS_PATH loop prevention)
>> >>>
>> >>> First, are the above designs correct?
>> >>>
>> >>> And second, what is the proper way to setup "Two sites, same ASN, with
>> >>> backdoor (backup-only) link"? I put it into GNS3, and I can't get the
>> >>> traffic to prefer the MPLS backbone instead of the backup (iBGP) link.
>> >>> The
>> >>> PE routers both prefer the eBGP link towards the internal network, and
>> >>> neither will accept the (preferred) iBGP ad from their peer PE router.
>> >>> It
>> >>> works fine going the reverse -- backup link works great, and if the
>> >>> backup
>> >>> link goes down the MPLS core is used.
>> >>>
>> >>> The only way I can figure to get the desired "MPLS primary, backdoor
>> link
>> >>> is backup" behavior is to do some kludge between the CE routers so
>> that
>> >>> they only advertise the routes to each other if the MPLS network is
>> down.
>> >>>
>> >>> Is the only time you want to use BGP SoO when all your sites are on
>> the
>> >>> same AS (forcing allowas-in or as-override), and some are multihomed?
>> >>> Otherwise, won't BGP AS_PATH loop prevention prevent the looping?
>> >>>
>> >>> Very confused, appreciate your input!
>> >>>
>> >>> Keller Giacomarro
>> >>> keller.g_at_gmail.com
>> >>>
>> >>>
>> >>> Blogs and organic groups at http://www.ccie.net
>> >>>
>> >>>
>> _______________________________________________________________________
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>> >>>
>> >>>
>> >>>
>> >>>
>> >>>
>> >>>
>> >>>
>> >>>
>> >>
>> >>
>> >> --
>> >> Thanks & Regards,
>> >> Gaurav Thukral
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >
>>
>>
>> --
>> Thanks & Regards,
>> Gaurav Thukral
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
>> _______________________________________________________________________
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Received on Mon Oct 08 2012 - 02:25:29 ART