Re: OT: GS Archives Search

Marko,

My assumption was MPLS VPNs, i think we both know that.

On Mon, Mar 26, 2012 at 7:42 PM, Marko Milivojevic <markom_at_ipexpert.com>wrote:

> Just to clarify - I never said RT was part of the bestpath selection. I did
> put the "iPhone disclaimer" (twice, actually because I knew someone is
> going to jump at the short explanation), which means I was on the road and
> the example may not have been the clearest possible. Now I'm not on iPhone,
> so allow me to clarify my original answer, by building on yours and
> Narbik's.
>
> What you wrote is not by a long shot incorrect, but it's also not the exact
> reason *why* RTs are not used as a differentiator or *why* we have RDs and
> RTs to begin with, which is something many students struggle with. We all
> know what they do, but I like to explain *why* certain things behave the
> way do.
>
> There are two reasons for having address space expanded from the original
> 32 bits:
>
> 1. Allow for overlapping addresses in different VPNs
> 2. Allow for iBGP load-sharing within the same VPN
>
> The main reason why the route distinguisher is *not* used to associate
> routes with VPNs is the lack of flexibility that would provide. Basically,
> if we used only RD for VPN membership in a fashion similar to
> route-targets, the possible logical topologies would be limited without
> extensive filtering (anything is doable with the ample amount of
> route-maps). Using communities to associate routes with VPNs allows for a
> more architectural and scalable approach to route association with VPNs
> than using a single entity like RD ever does.
>
> Thus, we have two different entities we have to deal with: RD and RT and
> a fundamental reason for having a separation between RD and RT is to allow
> for topology flexibility and uniqueness of prefixes. The traditional book
> example you gave with overlapping customer addresses is just one of the
> reasons for RDs. Another, also very common is the inability to use iBGP
> load-balancing when using same RDs within the same VPN, when using
> route-reflectors. The reason is, of course, that RRs advertise only the
> best prefix. I believe there are some recent RFCs that address this issue,
> but I'm yet to see the working code.
>
> Narbik: VRFs work just fine without RDs. It's MPLS VPNs that don't and
> there is a difference between the two :-)
>
> --
> Marko Milivojevic - CCIE #18427 (SP R&S)
> Senior CCIE Instructor - IPexpert
>
> --
> Marko Milivojevic - CCIE #18427 (SP R&S)
> Senior CCIE Instructor - IPexpert
>
> On Mon, Mar 26, 2012 at 18:27, Brian McGahan <bmcgahan_at_ine.com> wrote:
>
> > Personally that seems overly confusing to me. Yes Route Targets are an
> > attribute of the route, but that attribute is not part of the BGP
> Bestpath
> > Selection. I'm not sure how it ties together. It's simpler to think of
> it
> > this way:
> >
> > It's given that customers of a Service Provider will have overlapping IP
> > addressing in their VPNs, e.g. you will have more than two customers who
> > use the 10.0.0.0/8 network. The RD is how you tell them apart. If you
> > have customer "A" with RD "A" and customer "B" with RD "B" the routes "A:
> > 10.0.0.0/8" and "B:10.0.0.0/8" become unique. This is all the RD does.
> >
> > The Route Target tells you which VRF table the route belongs to. You
> have
> > to separate the two attributes because sometimes you want the same route
> to
> > belong to multiple VRF tables. This is common in what's known as
> "Central
> > Services VPNs". For example if the Service Provider hosts email for
> > customers, that route to the mail server would have to be in the routing
> > table of multiple customers. This doesn't break the rule of the route
> > having to be unique though, which is what the RD does.
> >
> > Like I said you may be able to find more clarification in this video:
> > http://goo.gl/Y0imB.
> >
> > Brian McGahan, CCIE #8593 (R&S/SP/Security)
> > bmcgahan_at_INE.com
> >
> > Internetwork Expert, Inc.
> > http://www.INE.com
> >
> > -----Original Message-----
> > From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> > Marko Milivojevic
> > Sent: Monday, March 26, 2012 6:00 PM
> > To: Yemi Salau
> > Cc: ccielab_at_groupstudy.com
> > Subject: Re: OT: GS Archives Search
> >
> > Simple reason - prefixes are passed on through the bestpath selection
> > process where the best one is chosen based on attributes. RT is a
> > community, which is an attribute. This means that given two prefixes with
> > different RTs would be treated as equals when it comes to bestpath
> > selection. With RD we extend the prefix space to 86 bits and then use
> those
> > for comparison instead of 32bit ones.
> >
> > [ iPhone, brevity, etc disclaimer :-) ]
> >
> > --
> > Marko Milivojevic - CCIE #18427
> >
> > :: This message was sent from a mobile device. I apologize for errors and
> > brevity. ::
> >
> > On Mar 26, 2012, at 14:44, Yemi Salau <salauolayemi_at_yahoo.co.uk> wrote:
> >
> > > Thanks Marko, reading RFC 4364, I was trying to figure out why RT
> > > couldn't
> > do the same job of RD for uniquely separating VPN-IPv4 routes within the
> > provider MPLS cloud. I'll watch your video when I get home. Many Thanks.
> > >
> > > From: Marko Milivojevic <markom_at_ipexpert.com>
> > > To: Yemi Salau <salauolayemi_at_yahoo.co.uk>
> > > Cc: ccielab_at_groupstudy.com
> > > Sent: Monday, 26 March 2012, 16:25
> > > Subject: Re: OT: GS Archives Search
> > >
> > >
> > > Yemi,
> > >
> > > I'm not sure about the Archive search, but I can certainly help you
> > > with RD
> > and RT. Almost two years ago I hosted a free online training session
> > called "MPLS 101". You can find it, together with all other recorded
> > vLectures on many other subjects here:
> > >
> > > http://bit.ly/vLecture
> > >
> > > Please go ahead and watch it and if you have any questions feel free
> > > to ask
> > them here.
> > >
> > > --
> > > Marko Milivojevic - CCIE #18427 (SP R&S) Senior CCIE Instructor -
> > > IPexpert
> > >
> > > On Mon, Mar 26, 2012 at 10:18, Yemi Salau <salauolayemi_at_yahoo.co.uk>
> > wrote:
> > > Guys,
> > >
> > > I remember a time where I was able to search the GS archives for
> > > stuffs. Is
> > this still available today? I want to search out some stuffs on RD vs RT.
> > >
> > > Yemi
> > >
> > >
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> > >
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-- 
*Narbik Kocharians
*CCSI#30832, CCIE# 12410 (R&S, SP, Security)
*www.MicronicsTraining.com* <http://www.micronicstraining.com/>
Sr. Technical Instructor
YES! We take Cisco Learning Credits!
A Cisco Learning Partner
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