[nsp] Stub Area design question
Howard C. Berkowitz
hcb at gettcomm.com
Sun Jul 18 17:08:44 EDT 2004
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)
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