Subject: [RRG] LISP question

From: Robert Raszuk <[email-masked]>
Date: Thu, 06 Dec 2007 15:22:38 -0800

In the interest of saving time tomorrow I would like to ask for some 
clarification reg how large corporate enterprise sites (those who run 
full table of BGP today) would auto discover ITRs ?

Their network maybe multi-as as well as multi-area. The answer from the 
audience of LISP tutorial was that EIDs-subnets would be carried in 
those networks in BGP to attract traffic to ITRs. But on the other hand 
we know that there can be much more EID subnets (I think I heard 3 fold 
estimate) then there is today IP prefixes in full BGP table ... not too 
mention 10^10 long term number.

What would be the benefit and driver for such large corporate sites to 
install ITRs ?

Thx,
R.

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From: Tony Li <[email-masked]>
Date: Thu, 6 Dec 2007 15:43:49 -0800

On Dec 6, 2007, at 3:22 PM, Robert Raszuk wrote:

> In the interest of saving time tomorrow I would like to ask for  
> some clarification reg how large corporate enterprise sites (those  
> who run full table of BGP today) would auto discover ITRs ?
>
> Their network maybe multi-as as well as multi-area. The answer from  
> the audience of LISP tutorial was that EIDs-subnets would be  
> carried in those networks in BGP to attract traffic to ITRs. But on  
> the other hand we know that there can be much more EID subnets (I  
> think I heard 3 fold estimate) then there is today IP prefixes in  
> full BGP table ... not too mention 10^10 long term number.

Most likely, the ITRs would be co-located with the exact same routers  
that originated default into the IGP.

> What would be the benefit and driver for such large corporate sites  
> to install ITRs ?

Ability to advertise locators on a per-host basis and perform inbound  
TE.  Ability to effectively get current PI capabilities without  
actually justifying a PI prefix.

Tony

---

From: David Meyer <[email-masked]>
Date: Thu, 6 Dec 2007 16:26:40 -0800

Robert,

> clarification reg how large corporate enterprise sites (those who run full 
> table of BGP today) would auto discover ITRs ?
>
> audience of LISP tutorial was that EIDs-subnets would be carried in those 
> networks in BGP to attract traffic to ITRs. But on the other hand we know 
> that there can be much more EID subnets (I think I heard 3 fold estimate) 
> then there is today IP prefixes in full BGP table ... not too mention 10^10 
> long term number.

	As you point out, today an enterprise finds its "best
	exit" by using the (BGP) routes that it learns from its
	various providers and distributing internally them with
	iBGP. If the ITRs are sitting on the edge of the site's
	network, this won't work since packets will be routed
	internally following default. A packet must follow
	default because there no information about the best exit
	for a given EID prefix.  

	So in order to preserve the behavior you are describing,
	the packet needs to be routed over the RLOC space as
	early as possible. This corresponds to placing the ITRs
	deeper in the site (closer to the source hosts). Since
	the ITR will encapsulate the packet (and the outer header
	destination address will be a locator), it will find the
	best exit just as it does today.  

> What would be the benefit and driver for such large corporate sites to 

	Finer-grained control over exit policy without having to
	propagate global state inside your enterprise.

	Dave

---

From: Robert Raszuk <[email-masked]>
Date: Thu, 06 Dec 2007 22:33:52 -0800

Dave,

> 	propagate global state inside your enterprise.

But what you and Tony replied are not the mirror replacement of today's 
optimal routing to dst prefix ... this is optimal closest exit strategy 
or in other words ability to find closest ITR. I am not aware about an 
ITR to ITR relays for optimal routing towards destination in any of the 
current proposals :).

I wonder why so many networks today still run full BGP inside rather 
then just injecting whole bunch of even anycast defaults into their IGP 
to ASBRs. It would be nice to have such public data :).

Perhaps engineers of such networks should speak up on their reasons as I 
can not disclose neither the names nor their internal network 
architecture fundamentals ...

Cheers,
R.

---

From: Dino Farinacci <[email-masked]>
Date: Fri, 7 Dec 2007 00:52:42 -0800

>>> What would be the benefit and driver for such large corporate  
>>> sites to install ITRs ?
> But what you and Tony replied are not the mirror replacement of  
> today's optimal routing to dst prefix ... this is optimal closest  
> exit strategy or in other words ability to find closest ITR. I am  
> not aware about an ITR to ITR relays for optimal routing towards  
> destination in any of the current proposals :).

Robert, I think you didn't get Dave's point. Let me restate a bit:

o If you put the ITRs at the edges of a source site, then you route on  
the EID
   from the source host to the edge LISP router. In this case you get  
to exit
   based on the default route, which is through the closest LISP router.

o When you want to get shortest path to the destination EID based on  
the locators
   the destination site has provide to the source site, you put the  
ITRs, say in
   the middle of the source site. When you do that, the packet is  
routed based
   on EID to the ITRs. But from that point on you route based on the  
locator.
   So the exit router the packet travels through is based on the  
shortest path
   to the destination locator.

o If you put the ITRs directly connected to the sources, one or more  
on each
   source subnet, you get destination based shortest paths just like  
you do today.

But getting what you do today is at the cost of carrying full routes  
in the source site. So if you place your ITRs at the core of the  
source site, you can get good paths to each external destination  
without carrying full routes *throughout* your source site.

So it will actually be less cost to do what you are doing today. You  
don't need full routes everywhere, and you probably won't need iBGP  
everywhere as well.

> then just injecting whole bunch of even anycast defaults into their  
> IGP to ASBRs. It would be nice to have such public data :).

They want shortest paths to reduce latency and packet delay.  
Especially the big content providers where competition is fierce for  
low latency and giving the user the best user experience.

> Perhaps engineers of such networks should speak up on their reasons  
> as I can not disclose neither the names nor their internal network  

Yes, many would welcome that.

Dino