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UDP port 500 is the ISAKMP port for establishing PHASE 1 of IPSEC tunnnel. 500 designs VPN-GW1-----nat rtr-----natrtr-----VPNGW2 If two vpn routers are behind a nat device or either one of them, then you will need to do NAT traversal which uses port 4500 to successfully establish the complete IPEC tunnel over NAT devices