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isakmp port changes

Can the ISAKMP UDP port 500 change at any time? I have just resolved a problem where the ISAKMP destination port had changed to UDP/9.. It was UDP/500 three days ago? I am under the impression that this is fixed in the router and should not change at all. Any idea's on this. I have resolved tghe issue by allowing the UDP/9 port in and all s working. Any idea's on how to change it back to UDP/500?

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Re: isakmp port changes

It shouldn't. What device is listening for isakmp on udp 9?

Nat traversal will use udp4500, but still, the initial isakmp is still udp 500 (in the isakmp conversation, mutual support of nat-t is established and then used if necessary or so configured).

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