Cisco CRS-1 - Carrier Routing System Router Configuration Manual page 117

Implementing Internet Key Exchange Security Protocol on Cisco IOS XR Software
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Policy Creation
You can create multiple IKE policies, each with a different combination of parameter values. For each
policy that you create, assign a unique priority (1 through 10,000, with 1 being the highest priority).
You can configure multiple policies on each peer—but at least one of these policies must contain exactly
the same encryption, hash, authentication, and Diffie-Hellman parameter values as one of the policies
on the remote peer. (The lifetime parameter need not necessarily be the same; see details in the
Peer Agreement for Matching Policies" section on page
OL-20382-01
Information About Implementing IKE Security Protocol Configurations for IPSec Networks
MD5 has a smaller digest and is considered to be slightly faster than SHA-1. A demonstrated
successful (but extremely difficult) attack has been demonstrated against MD5; however, the HMAC
variant used by IKE prevents this attack.
The authentication method has three options: RSA signatures, RSA encrypted nonces, and preshared
keys.
RSA signatures provide nonrepudiation for the IKE negotiation. (You can prove to a third party
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after the fact that you did indeed have an IKE negotiation with the remote peer.)
RSA signatures allow the use of a CA. Using a CA can dramatically improve the manageability
and scalability of your IPSec network. Additionally, RSA signature-based authentication uses
only two public key operations, whereas RAS encryption uses four public key operations,
making it costlier in terms of overall performance.
You can also exchange the public keys manually, as described in the
RSA Keys" section on page
– RSA encrypted nonces provide repudiation for the IKE negotiation (you cannot prove to a third
party that you had an IKE negotiation with the remote peer).
RSA encrypted nonces require that peers possess each other's public keys but do not use a
certification authority. Instead, two ways exist for peers to get each other's public keys:
During configuration, you manually configure RSA keys (as described in the
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Configuring RSA Keys" section on page
If your local peer has previously used RSA signatures with certificates during a successful IKE
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negotiation with a remote peer, your local peer already possesses the remote peer's public key.
(The peers' public keys are exchanged during the RSA-signatures-based IKE negotiations, if
certificates are used.)
Preshared keys are clumsy to use if your secured network is large, and they do not scale well
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with a growing network. However, they do not require use of a certification authority, as do RSA
signatures, and might be easier to set up in a small network with fewer than ten nodes. RSA
signatures also can be considered more secure when compared with preshared key
authentication.
The Diffie-Hellman group identifier has three options: 768-bit, 1024-bit Diffie-Hellman, and
1536-bit Diffie Hellman.
The 1024-bit Diffie-Hellman and 1536-bit Diffie Hellman options are harder to crack but require
more CPU time to execute.
The lifetime of the security association can be set to any value.
As a general rule, the shorter the lifetime (up to a point), the more secure your IKE negotiations.
However, with longer lifetimes, future IPSec security associations can be set up more quickly. For
more information about this parameter and how it is used, see the command description for the
lifetime command.
Cisco IOS XR System Security Configuration Guide for the Cisco CRS-1 Router
121.
121).
109.)
"Manually Configuring
"Manually
"IKE
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