[c-nsp] asymmetric multihoming & nat

[Adam Greene]

Hi guys,

I have a multihomed customer who receives full BGP routes from both us 
and another provider and load balances between the two connections. 
Things are working fine until the traffic becomes asymmetric (i.e. 
inbound through one provider, outbound through the other).

The block they are announcing to their providers is NATed on their BGP 
router. In other words, all their internal hosts are on private IP 
space. The internal interface is designated "ip nat inside" and both WAN 
interfaces are designated "ip nat outside". The actual NAT 
configurations do not reference any interfaces, just pools.

Could the NAT be prohibiting asymmetric traffic in this case? i.e. if 
the inbound traffic is NATed coming in on one interface, will the router 
refuse to NAT the outbound traffic through the other interface?

If the NAT is the problem, I suppose they could do the NAT on a loopback 
interface instead ... but I understand that the traffic will all be 
process-switched if we do that, and performance will probably suffer.

Thanks for your insight,
Adam