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

[Gregory Boehnlein]

> I'm curious to see what everyone's take is on handling the addressing
> of customer-facing DS1s.  Rather than provision a /30 per customer and
> waste IP space I'm planning on using IP unnumbered to a loopback for
> the vast majority of our most basic DS1 customers.  They'll get assigned 1
> IP out of the pool and if they request (and pay for) any more then I'll
> static route their allocation to their pool IP.  We'll also request
> that they only use the statically-routed subnet for server assignments and
> not PAT their pool IP; that way they can be migrated to our LRE
> solution with minimal IP and DNS changes as LRE becomes available in their
> local CO.

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.

> One of my concerns is QoS.  We'll be offering QoS-enabled VoIP over
> these DS1s in some cases.  Can I still use service-policies on physical
> interfaces or would it have to be on the loopback and apply to all
> users?  I have not tried QoS and IP unnumbered yet.  Perhaps VoIP
> circuits will have to be addressed with /30s (or /31s since we're
> managing the CE in VoIP installations).

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

And the routing statement:

ip route X.X.X.X 255.255.255.248 Serial10/1/0/3:0 name CustomerRouteA

 
> I know that IP unnumbered works in general; I'm just wondering if
> anyone has already done it in production and ran into problems?  My goal
is to
> not waste IP space on network and bcast addresses where possible.  No,
> I'm not going to assign /31s; customers just won't be able to
> comprehend
> that, especially when the IOS through our a warning message when you
> assign a /31 to an interface.  I use /31s internally but not for
> customer links.  IP unnumbered wastes the least amount of IP space.
> Since we're targeting our services at the low-end of the SMB market who
> most likely it already on CATV or RBE DSL and share a bcast domain with
> their neighbors already, I don't envision IP unnumbered to be a big
> deal.  I'm looking for guidance though.  Thoughts?

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.