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Chapter 15 IPSec VPN
NBG-460N User’s Guide
220
IPSec router use in the IKE SA. In main mode, this is done in steps 1 and 2, as
illustrated below.
Figure 137 IKE SA: Main Negotiation Mode, Steps 1 - 2: IKE SA Proposal
The NBG-460N sends a proposal to the remote IPSec router. Each proposal
consists of an encryption algorithm, authentication algorithm, and DH key group
that the NBG-460N wants to use in the IKE SA. The remote IPSec router sends the
accepted proposal back to the NBG-460N. If the remote IPSec router rejects the
proposal (for example, if the VPN tunnel is not configured correctly), the NBG-
460N and remote IPSec router cannot establish an IKE SA.
Note: Both routers must use the same encryption algorithm, authentication algorithm,
and DH key group.
See the field descriptions for information about specific encryption algorithms,
authentication algorithms, and DH key groups. See Section 15.6.3 on page 220
for more information about DH key groups.
15.6.3 Diffie-Hellman (DH) Key Exchange
The NBG-460N and the remote IPSec router use a DH key exchange to establish a
shared secret, which is used to generate encryption keys for IKE SA and IPSec SA.
In main mode, the DH key exchange is done in steps 3 and 4, as illustrated below.
Figure 138 IKE SA: Main Negotiation Mode, Steps 3 - 4: DH Key Exchange
The DH key exchange is based on DH key groups. Each key group is a fixed
number of bits long. The longer the key, the more secure the encryption keys, but
also the longer it takes to encrypt and decrypt information. For example, DH2
keys (1024 bits) are more secure than DH1 keys (768 bits), but DH2 encryption
keys take longer to encrypt and decrypt.
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