Profile: Margaret Rouse
|IKE negotiation sends and receives messages using UDP, listening on port 500. This can be a problem if you have a firewall in front of your VPN router or are trying to establish an IPsec client connection through a firewall.
Michael J. Martin, IPsec VPN router configuration: The ISAKMP policy
I wish I had read this earlier — Michael says “Remember that IKE is a protocol that supports ISAKMP — ISAKMP makes the rules, and IKE plays the game.”