IKE traffic uses UDP port 500. This is the default port for IKE traffic. If you want to change the port, you must change it on both the client and server.
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Introduction
Internet Key Exchange (IKE) is a layer 7 (application layer) protocol that uses UDP port 500. IKE uses UDP because it needs the low-overhead afforded by a connectionless protocol. The use of UDP also allows IKE to use NAT-T (NAT Traversal), which allows IKE to work through NAT devices.
VPN Client
The short answer is that the VPN Client uses UDP port 500 for IKE traffic from the VPN Client to the VPN Server.
IKE is a security protocol that is used to set up a secure communication channel between two devices. IKE uses a variety of methods to authenticate devices and to set up and manage secure communication channels. One of the methods that IKE uses is UDP port 500.
When a VPN Client wants to set up a secure communication channel with a VPN Server, it sends an IKE message to the server on UDP port 500. The IKE message contains information about the client and the proposed security parameters for the communication channel. The VPN Server responds with an IKE message of its own, and if both messages are acceptable, a secure communication channel is set up between the two devices.
UDP port 500 is just one of the ports that can be used for IKE traffic; other commonly used ports include UDP port 4500 and TCP port 10000.
VPN Server
IKE uses UDP port 500 for Phase 1 traffic and IP Protocol 50 for Phase 2 traffic.
IKE Traffic
UDP port 500 is used for IKE traffic.
Conclusion
In conclusion, IKE traffic from VPN Client to Server uses UDP port 500.