Problems with removing NAT from a network

Michael Smith michael at
Wed Jan 5 20:51:23 CST 2011

The devil's in the details (obviously), and someone that reads into the
scenario better than me might have a more direct suggestion, but...

I'd start by moving the NAT at least one hop into the AS so that routing
symmetry can be enforced there.  This allows for multi-homing (asymmetric
routing at the edge) without (before) dealing with the NAT issue (if there
is one at that point).

On Jan 5, 2011 9:39 PM, "ML" <ml at> wrote:

I've got a customer that is looking to multihome with upstreams in two POPs.
 Currently they multihome in one POP and utilize a single edge router for
some one to one NAT and some PAT for their users.

Before they turn up the BGP peer in the new POP I've advised them to abolish
NAT once and for all in order to avoid issues with non-stateful NAT between
network edges and possible asymmetric routing of their Internet traffic.

The PAT can be removed easily enough.  The tricky part is the one-one NAT.
They have quite a few systems which have 1918 IPs which they claim "cannot
be changed". At least not without some painful rebuilds of criticals systems
which have these IPs deeply embedded in their configs.

Has anyone here had to fix this kind of problem before? Is there a solution
that would allow NAT to offloaded to a smaller device hanging off each edge
router that can communicate state between each other in case traffic is
asymmetrically routed?

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