[c-nsp] DS1 provisioning using IP Unnumbered vs /30s

[Gert Doering]

Hi,

On Fri, Feb 06, 2009 at 05:08:47PM -0600, Justin Shore wrote:
> Gregory Boehnlein wrote:
> >This is very similar to what we do. While we have several customers that
> >need larger subnets, the majority of our customers are using IP Unnumbered.
> >
> >In some cases, we will provide the customer a /29 if they need additional
> >external IP addresses, but the configuration on their router uses the low 
> >IP
> >in the /29 as it's loopback interface. We send our default route out of the
> >interface, rather than to the remote gateway IP, so if we change the ip of
> >the loopback on our side, we do not need to adjust anything for the
> >customer.
> 
> That's not a bad idea.  Though wouldn't pointing a default at an 
> interface force it to ARP constantly?  Several hundred CEs ARPing 
> non-stop could be a load issue on your PE.

There's no ARP on serial links (HDLC or PPP).  There's only "me" and "you".

So if you point a route to the interface, the router will stuff it in, and
the other router will receive it.

You need to be careful about ATM interfaces: it depends on how you configure
them.  If it's PPPoA, or AAL5 SNAP ("direct IP over ATM"), this will
work.  If you involve Ethernet, like with RBE (route-bridged encapsulation)
or with bridge-groups over ATM, you have ARP, and should specify a 
next-hop router.

> I'm not too worried about the loopback's IP changing.  I only put a /24 
> on the loopback to get us started.  I'll load it with customer DS1s 
> until I get to around 90% IP utilization and then create another 
> loopback with another /24 or perhaps a /23 this time around.  I padded 
> the front of the subnet sufficiently to allow for future HA projects and 
> the unforeseeable.  I can't foresee ever needing to change the IP on a 
> loopback once I have it in production.

Imagine customers moving to other routers.

[..]
> >ip route X.X.X.X 255.255.255.248 Serial10/1/0/3:0 name CustomerRouteA
> 
> So you'd prefer to route a larger customer assignment to the the 
> CE-facing interface instead of to their assigned IP unnumbered address?

Most definitely.

(That way, when the interface goes down, the route is properly withdrawn.
Otherwise, as long as you have route to the customer "IP unnumbered" 
address in your routing table, your PE router will still have the route
active, and will announce it in its IGP - leading to routing loops)

gert
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USENET is *not* the non-clickable part of WWW!
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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