[nsp] Stub Area design question

[Howard C. Berkowitz]

At 3:53 PM -0400 7/18/04, Dan Armstrong wrote:
>I have a desgin question about IGP routing protocols in a stub area.
>
>Picture a generic service provider network.  Several access routers to which
>customers are attached.  All of them dual homed up to 2 distribution layer/
>metro core routers.  All of the access routers are in the same ospf area.
>Each city (usually comprised of 2 Metro POPs) is 1 OSPF area.

Do the customers just default to you, or do you do BGP or even IGP 
peering with them?

>
>The area in which the access routers are is an nssa totally stub area.  I just
>want to inject a default route from each of the distribution layer routers
>down to the access routers.  I want to advertise the C and S routes UP from
>the access routers, I redist static subnets, redist conn subnets and we're
>good to go.

I'm losing track of where you are redistributing. If it's on the 
distribution router, I'm wondering if you really should be running 
OSPF at all at the access tier, but rather static routing (with 
appropriate floating statics for failover).

Remember that you are either going to have to assign PA address space 
to the customer, or know what the customer needs. That information is 
going to go into a database somewhere.  If you look at a presentation 
I did at NANOG, http://www.nanog.org/mtg-9811/ppt/berk/index.htm, you 
will see the skeleton of how you could automatically generate the 
static routes (and other good things) for the access and distribution 
tier.  Doing things this way takes away one of the common objections 
to static routing: "it's too maintenance-intensive."  (Shameless plug 
-- I go into greater detail in my book, _Building Service Provider 
Networks_ [Wiley]).

>
>The only thing about this design that bothers me is that since all of the
>access routers are in the same area, they all get each other's intra area
>routes.  Access router A can only ever reach access router B through the
>distribution layer anyway.  Each access router does not care what blocks are
>anchored on other access routers, they just need the default route(s). 
>
>Should each access router be in it's own area?  Would having a crapload of
>ospf areas on the distribution layer routers knock some limit over? 
>
>Could I write a bunch of messy route maps to handle this?
>
>Am I missing something really dumb and obvious?

To me, the fundamental question is whether dynamic routing buys you 
anything at the access tier. Inserting static routes to the customer 
blocks in the distribution router, and redistributing static, may be 
all you need.

I say "may", because I don't know if there's any dynamic routing 
exchange between your access routers and the customer routers. If 
there is, then the problem gets more complex.
-- 

Howard C. Berkowitz
Chief Technology Officer
Gett Communications
5012 25th Street South
Arlington VA 22206

(703)998-5819 voice
(703)998-5058  fax (alas, sometimes poorly operated by "helpful" cat)