Nat redunduncy

Unanswered Question
Aug 12th, 2008

Hi Experts,

I have two internet connections from two different SPs. and they have assigned address block from their pool. I am using SP-B for back up and SP-A as a primary one. I want to use NAT but the problem is when link towards SP-A goes down SP-B will take over, in this situation how can i configure backup NAT for the addresses of SP-B? and automatic take over will took place?

I have this problem too.
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Loading. Wed, 08/13/2008 - 01:02

Hi Shrikar,

AFAIK this is not possible.Consider your local address when translated to global address. This global address is for SP-A. When the link fail still the local address will translate to SP-A global addr. Packets would be routed to SP-B and there would be no return path with SP-B. I have considered that you are using floating routes. If you make use of BGP then i think it would be possible.

Experts Correct me if I am wrong.



Nikhil E.

dzanolari Wed, 08/13/2008 - 03:56

As far as I understand, both ISPs assign you their address space. So you can use two links, but each of them has to use an IP address assigned of the respective subnet. IP addressed assigned by ISP A will always route to him and cannot reach active components assigned such an address over ISP B and vice versa. Furthermore you will have no fail-over for existing sessions. If one link fails, you can route traffic over the other link, but all connections need to be restablished, as the source address for the server at the far end changes.

If you need redundancy, you either can have two uplinks to _one_ ISP or you need to ask for an own AS (and use BGP), which two or more providers can connect to.

Keep in mind that even this won't solve the issue with the failing NAT device. While your uplink is redundant, the source-address would still change, thus estahlished sessions will fail. You need to have a redundant NAT device to overcome this.


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