Cisco ASA Series Cli Configuration Manual page 1561

Chapter 1 Configuring IPsec and ISAKMP
Table 1-2 IKEv2 Policy Keywords for CLI Commands (continued)
Command Keyword
group 1
2 (default)
5
14
19
20
21
24
lifetime integer value
(86400 =
default)
IKEv1 and IKEv2 each support a maximum of 20 IKE policies, each with a different set of values.
Assign a unique priority to each policy that you create. The lower the priority number, the higher the
priority.
When IKE negotiations begin, the peer that initiates the negotiation sends all of its policies to the remote
peer, and the remote peer tries to find a match. The remote peer checks all of the peer's policies against
each of its configured policies in priority order (highest priority first) until it discovers a match.
A match exists when both policies from the two peers contain the same encryption, hash, authentication,
and Diffie-Hellman parameter values. For IKEv1, the remote peer policy must also specify a lifetime
less than or equal to the lifetime in the policy the initiator sent. If the lifetimes are not identical, the ASA
uses the shorter lifetime. For IKEv2 the lifetime is not negotiated but managed locally between each
peer, making it possible to configure lifetime independently on each peer. If no acceptable match exists,
IKE refuses negotiation and the SA is not established.
There is an implicit trade-off between security and performance when you choose a specific value for
each parameter. The level of security the default values provide is adequate for the security requirements
of most organizations. If you are interoperating with a peer that supports only one of the values for a
parameter, your choice is limited to that value.
Note
New ASA configurations do not have a default IKEv1 or IKEv2 policy.
To configure IKE policies, in global configuration mode, use the crypto ikev1 | ikev2 policy priority
command to enter IKE policy configuration mode.
You must include the priority in each of the ISAKMP commands. The priority number uniquely
identifies the policy and determines the priority of the policy in IKE negotiations.
To enable and configure IKE, complete the following steps, using the IKEv1 examples as a guide:
Meaning
Group 1 (768-bit)
Group 2 (1024-bit)
Group 5 (1536-bit)
120 to 2147483647
seconds
Description
Specifies the Diffie-Hellman group identifier, which the
two IPsec peers use to derive a shared secret without
transmitting it to each other.
The lower the Diffie-Hellman group number, the less CPU
time it requires to execute. The higher the Diffie-Hellman
group number, the greater the security.
The AnyConnect client supports DH group 1, 2, and 5 in
non-FIPS mode, and groups 2 and only in FIPS mode.
AES support is available on security appliances licensed for
VPN-3DES only. To support the large key sizes required by
AES, ISAKMP negotiation should use Diffie-Hellman
(DH) Group 5.
Specifies the SA lifetime. The default is 86,400 seconds or
24 hours. As a general rule, a shorter lifetime provides more
secure ISAKMP negotiations (up to a point). However, with
shorter lifetimes, the ASA sets up future IPsec SAs more
quickly.
Cisco ASA Series CLI Configuration Guide
Configuring ISAKMP
1-11