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IKE, is the initial negotiation phase, in which two VPN endpoints agree on methods which will be used to provide security for IP traffic. IKE helps in  managing connections, with the help of Security Associations,

The other important part is IP data in transfer, using encryption and authentication methods agreed during IKE negotiation. This can be with help of  protocols like IPsec protocols ESP, AH, or combination of both.

The flow of events is mentioned below:

  •     IKE negotiates how IKE should be protected
  •     IKE negotiates how IPsec should be protected
  •     IPsec moves data in the VPN

IKE,Internet Key Exchange:

The two things which are required are encryption and authentication algorithms, and the corresponding keys. The Internet Key Exchange protocol, IKE, is used as a method to distribute "session keys", and providing a way for the VPN endpoints to decide how to protect data.

IKE has three main tasks:

  • Provide a means for the endpoints to authenticate each other
  • Establish new IPsec connections (create SA pairs)
  • Manage existing connections

IKE Negotiation:

The flow of events are mentioned below:

IKE Phase-1

    Negotiate how IKE should be protected

IKE Phase-2

    Negotiate how IPsec should be protected

Derive some fresh keying material from the key exchange in phase-1, to provide session keys to be used in the encryption and authentication of the VPN data flow

IKE Phase-1 - IKE Security Negotiation

An IKE negotiation is performed in two phases. The first phase, phase-1, is used to authenticate the two VPN gateways or VPN Clients to each other, by confirming that the remote gateway has a matching Pre-Shared Key.

As you do not want to publish negotiation in plaintext, we first agree upon a way of protecting the rest of the IKE negotiation. This process is carried by the initiator sending a proposal-list to the responder. When this has been done, and the responder accepted one of the proposals, we try to authenticate the other end of the VPN to make sure it is who we think it is, as well as proving to the remote gateway that we are who we are.

Authentication can be accomplished through Pre-Shared Keys, certificates or public key encryption. Pre-Shared Keys is the most common authentication method today.

Core issue

The Internet Security Association and Key Management Protocol (ISAKMP) Phase 1 configurations do not match on the PIX/ASA Firewall peers.


To resolve this issue, check the isakmp policy command statements on both peers.

This is a sample output:

isakmp policy 10 authentication pre-share

isakmp policy 10 encryption des

isakmp policy 10 hash md5

isakmp policy 10 group 1

isakmp policy 10 lifetime 86400

isakmp policy 20 authentication pre-share

isakmp policy 20 encryption 3des

isakmp policy 20 hash sha

isakmp policy 20 group 2

isakmp policy 20 lifetime 86400

The PIX Firewall can have multiple Phase I policies, as shown in this example with policies 10 and 20.

The PIX Firewall goes through the list until the policy matches what is configured on the peer.

If no match is found, you will see debug errors about Phase 1 attributes mismatch.

For more information, refer to Configuring a Simple PIX-to-PIX VPN Tunnel Using IPSec and Configuring a Simple ASA-to-PIX VPN Tunnel

Problem Type


Product Family

Firewall - PIX 500 series

ASA Hardware & Software

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