Question about migrating to IPv6 with multiple upstreams.

William Herrin bill at
Tue Jun 14 17:28:28 UTC 2011

On Tue, Jun 14, 2011 at 1:04 PM, Ray Soucy <rps at> wrote:
> I think in the long term telling everyone to jump into the BGP table
> is not sustainable; and not operationally consistent with the majority
> of SMB networks.
> A better solution; and the one I think that will be adopted in the
> long term as soon as vendors come into the fold, is to swap out
> RFC1918 with ULA addressing, and swap out PAT with NPT; then use
> policy routing to handle load balancing and failover the way most
> "dual WAN" multifunction firewalls do today.
> Example:
> Each provider provides a 48-bit prefix;
> Internally you use a ULA prefix; and setup prefix translation so that
> the prefix gets swapped appropriately for each uplink interface.  This
> provides the benefits of "NAT" used today; without the drawback of
> having to do funky port rewriting and restricting incoming traffic to
> mapped assignments or UPnP.

Hi Ray,

There's a nuance here you've missed.

There are two main reasons for ULA inside the network:

1. Address stability (simplifies network management)
2. Source obfuscation (improves the depth of the security plan)

Option 1: Obfuscation desired.

ULA inside. NAT/PAT at both borders. You don't use prefix translation
here because prefix translation does little obfuscation: it has a 1:1
relationship with each individual host and still reveals the internal
routing structure.

Option 2: Stability, no obfuscation desired.

ULA inside, prefix translation at both borders.

Option 3: Neither stability nor obfuscation required.

GUA from one of the providers inside. Prefix translation to the other
provider for the connections desired out that border. Giving the hosts
real GUA addresses maximizes application compatibility.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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