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

[Justin Shore]

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.

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.

> We use outbound service policies on unnumbered interfaces just fine:
> 
> interface Serial10/1/0/3:0
>  description L3 OH/XXXX/XXXX to XXXXXXX channel 12
>  ip unnumbered Loopback0
>  no ip redirects
>  no ip unreachables
>  no ip proxy-arp
>  service-policy output llq
>  no fair-queue
>  down-when-looped
>  no clns route-cache

Excellent.  QoS was something that occurred to me as I was writing my 
original message.


> 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?

> I've been doing ip unnumbered since 1995 in production and it "just works".
> There are undoubtedly some routers out there that might have issues, but any
> Cisco device should be able to handle things just fine.

Very good.  On the CE side I really don't anticipate many problems.  I 
see this as more of a CE knowledge thing than anything.  I'm sure we'll 
have several customers that need something special.  I hope that for the 
  most part we can lump them all into an IP unnumbered design.

Thanks for the input
  Justin