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Pix PAT addresses posted 08/28/2000
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Title: Pix PAT addresses

Hi Guys,

I've got a scenario where 2 companies are using 1 Pix for inet access.  One, subnet A, enters the Pix on interface 1, the other, subnet B, enters the Pix on Interface 2 - or the "DMZ".  Obviously they are sharing the same public IP block.  This is a /27 range.  For address allocation, this block has been split down the middle - each company recieving 15 addresses.  Each company then sacrifices one address each for the upstream router and outside Pix address, leaving them with 14 each. 

I wanted to segment traffic outbound from the Pix so that should bandwidth consumption become an issue then each company would each be ensured 50% bandwidth on the Inet connection.  My thoughts were that at the upstream router I could utilize custom queueing to ensure this by creating access lists with a /28 mask i.e. access list 100 identifying the first 16 addresses in the block of 32, access list 101 identifying the next 16, then apply those lists to the queues (say queue 1 at 3000 bytes, queue 2 at 3000 bytes and the default queue at 1500) my theory being that if each subnet has its own queue then a third default queue would effectively always be empty (but still there to catch any possible oversights) and therefore traffic could be evenly distributed. I realize that all broadcasts and traffic bound for the Pix interface would be the burden of the company that had that address in its range, but that traffic is minimal and an accepted overhead.

However I was under the mistaken assumption that I would be able to configure more than one PAT address on the outside interface.  My intentions were to assign PAT address 1 - i.e. global (outside) 1 x.x.x.1-x.x.x.1 - to subnet A  and PAT address 2 - i.e. global (outside) 2 x.x.x.16-x.x.x.16 - to subnet B - and then match the appropriate nat-id's to the global commands. 

You cant assign more than one PAT address to an interface - the Pix clearly tells you one is already created and simply doesnt add the new.  Quite obviously if both inside interfaces have to utilize one PAT address then the whole equal distribution plan goes out the window.

Can anyone think of a config workaround to assigning multiple Global PAT addresses to the outside interface of a Pix - or an alternative way to achieving the goal?

Also any thoughts on this whole theory that I had on equal distribution (for example - if it would even work) would also be greatly appreciated.

Thanks

Jamie