[c-nsp] Interesting NAToverload issue

[nasir.shaikh at bt.com]

Hi,

I have a client who has moved their Microsoft Exchange servers to a service provider location (as part of a de-perimeterization strategy). These servers are reachable via the Internet. Thus, the client IP are NATted before they cross the corporate boundary.
There are about 45000 users.
Each user needs about 17-22 sessions (that's how MS Outlook works) and thus as many NAT entries
Therefore a NAT pool is used with overload.
It was working fine for more than a year now but suddenly the following phenomenon has been noticed.
- When a user session is being built up and he has let's say 10 NAT entries using the first IP in the NAT pool and the port numbers run out, the next IP in the NAT pool is used to complete the required number of sessions.
- Exchange server is apparently not happy with one client using 2 IP addresses and keeps (re-)building sessions untill all of them are using the same NATted IP. This can sometimes take upto 5 miniutes.

Is there a solution to this problem? There is one single destination global address. Is there a way to force the usage of the same IP from the NAT pool for all NAT requests from a particular source IP?
Platform is7206-vxr with NPE-G2

Thanks in advance


Nasir Shaikh 
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