[c-nsp] question about service provider network design

Nathan have.an.email at gmail.com
Tue Oct 21 11:47:22 EDT 2008

On Tue, Oct 21, 2008 at 4:16 PM, Dan Armstrong <dan at beanfield.com> wrote:
> We have a fairly similar design for our Metro Ethernet network.
> Our primary method of protection is STP(MST).  I've been thinking about
> this, and I can't come up with a reason why we even really need an IGP down
> to the edge PE devices?  Since it's all layer2 - the core switch/routers see
> all of the PE<>core links as Connected routes anyway - what's the point of
> bother pushing your IGP down there? It's just more needless routes.
> That leaves you with a very small IGP in your core.

The problem is that you are supposed to have redundant links between
routers. The way to have permanent links between routeurs in spite of
changing routes and falling interfaces is to establish communication
between loopbacks, and that is what LDP and iBGP - MPBGP do. Therefore
you need unfettered communication between the loopbacks of your
routers, PE routers included, therefore you need your loopbacks in
your IGP, therefore you need IGP on your PE routers.

I suppose you could somehow make the network function without it, but
you'd lose redundancy at the very least.


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