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Configuring the Cisco PIX Firewall for CA Site-to-Site

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Dec 28, 2001.

Chapter Description

This sample chapter explains how to configure Cisco Secure PIX Firewall certificate authority (CA) support for Internet Protocol Security (IPSec). After presenting an overview of the configuration process, the chapter shows you each major step of the configuration, including support tasks, IKE, and IPSec.

Task 1: Prepare for IPSec

Successful implementation of an IPSec network requires advance preparation before beginning configuration of individual PIX Firewalls. This section outlines how to determine network design details to configure a PIX Firewall for CA support.

You must plan in advance if you want to configure IPSec encryption correctly the first time and minimize misconfiguration. You should begin this task by defining the IPSec security policy based on the overall company security policy. Some planning steps follow:

Step 1

Determine CA server details—This includes variables such as the type of CA server to be used, the IP address, and the CA administrator contact information.

Step 2

Determine IKE (IKE phase one) policy—Determine the IKE policies between peers based on the number and location of IPSec peers.

Step 3

Determine IPSec (IKE phase two) policy—Identify IPSec peer details such as IP addresses and IPSec modes. You then configure crypto maps to gather all IPSec policy details together.

Step 4

Check the current configuration—Use the write terminal, show isakmp [policy], show crypto map, and many other show commands, which are covered later in this chapter in the "Test and Verify VPN Configuration" section.

Step 5

Ensure that the network operates without encryption—Ensure that basic connectivity has been achieved between IPSec peers using the desired IP services before configuring IPSec. You can use the ping command to check basic connectivity.

Step 6

Ensure that access lists are compatible with IPSec—Ensure that perimeter routers and the PIX outside interfaces permit IPSec traffic. Implicitly permit IPSec packets to bypass PIX access lists and conduits. In this step, you will need to enter the sysopt connection permit-ipsec command.


Step 1 and Step 2 are covered in detail in this chapter. The other steps are presented for review purposes. Refer to Chapter 6 for a detailed explanation of Steps 3 through 6.

Step 1: Determine CA Server Details

Successful implementation of an IPSec network requires advanced planning before beginning configuration of individual routers. This section outlines how to prepare for IKE and CA support.

Configuring CA is complicated. Having a detailed plan lessens the chances of improper configuration. Some planning steps include the following:

  • Determine the type of CA server to use—CA servers come in a multitude of configurations and capabilities. You must determine which one fits your needs before configuration. Requirements include, but are not limited to, Rivest, Shamir, and Adelman (RSA) key type; certificate revocation list (CRL) capabilities; and support for Registration Authority (RA) mode.

  • Identify the CA server IP address, host name, and URL—This information is necessary if you will be using Cisco Encryption Technology (CET) and Lightweight Directory Access Protocol (LDAP).

  • Identify the CA server administrator contact information—Arrange for your certificates to be validated if the process is not automatic.

The goal of these planning steps is to be ready for CA support configuration.

You need to have a CA available to your network before you configure CA. The CA must support Cisco's Public Key Infrastructure (PKI) protocol and the Simple Certificate Enrollment Protocol (SCEP).

Figure 7-1 shows an example of CA server details that you should gather before beginning the configuration. The diagram and table in Figure 7-1 illustrate the network topology and CA server details used in this chapter.

Figure 7-1 Determine CA Server Details

Step 2: Determine IKE (IKE Phase One) Policy

You should determine the IKE policy details to enable the selected authentication method, then configure that method. Having a detailed plan lessens the chances of improper configuration. Some planning steps include the following:

  • Determine the key distribution method—Determine the key distribution method based on the numbers and locations of IPSec peers. For small networks, you might wish to manually distribute keys. For larger networks, you might wish to use a CA server to support scalability of IPSec peers. You must then configure Internet Security Association Key Management Protocol (ISAKMP) to support the selected key distribution method.

  • Determine the authentication method—Choose the authentication method based on the key distribution method. The PIX Firewall supports either preshared keys, RSA encrypted nonces, or RSA signatures to authenticate IPSec peers. This chapter focuses on using RSA signatures.

  • Identify IPSec peer's IP addresses and host names—Determine the details of all the IPSec peers that will use ISAKMP and RSA signatures for establishing SAs. You will use this information to configure IKE.

  • Determine ISAKMP policies for peers—An ISAKMP policy defines a combination, or suite, of security parameters to be used during the ISAKMP negotiation. Each ISAKMP negotiation begins by each peer agreeing on a common (shared) ISAKMP policy. The ISAKMP policy suites must be determined in advance of configuration. You must then configure IKE to support the policy details you determined. Some ISAKMP policy details include the following:

    • Encryption algorithm

    • Hash algorithm

    • IKE SA lifetime

An IKE policy defines a combination of security parameters used during the IKE negotiation. A group of policies makes up a protection suite of multiple policies that enable IPSec peers to establish IKE sessions and establish SAs with minimal configuration.

Creating IKE Policies for a Purpose

IKE negotiations must be protected, so each IKE negotiation begins by each peer agreeing on a common (shared) IKE policy. This policy states which security parameters will be used to protect subsequent IKE negotiations.

After the two peers agree upon a policy, the security parameters of the policy are identified by an SA established at each peer. These SAs apply to all subsequent IKE traffic during the negotiation.

You can create multiple prioritized policies at each peer to ensure that at least one policy will match a remote peer's policy.

Defining IKE Policy Parameters

You can select specific values for each IKE parameter per the IKE standard. You choose one value over another, based on the security level you desire and the type of IPSec peer to which you will connect. There are five parameters to define in each IKE policy, as outlined in Table 7-1 and Figure 7-2. Table 7-1 shows the default values of each parameter, and Figure 7-2 shows the relative strengths.

Table 7-1 IKE Policy Parameters


Accepted Values



Message encryption algorithm






Message integrity (hash) algorithm

SHA-1 (HMAC variant)

MD5 (HMAC variant)




Peer authentication method

Preshared keys

RSA signatures



RSA signatures

Key exchange parameters (Diffie-Hellman group identifier)

768-bit Diffie-Hellman or

1024-bit Diffie-Hellman



768-bit Diffie-Hellman

ISAKMP-established SA's lifetime

Can specify any number of seconds


86,400 seconds (one day)

Figure 7-2 IKE Policy Example

You can select specific values for each ISAKMP parameter using the ISAKMP standard. You choose one value over another based on the security level you desire and the type of IPSec peer to which you will connect. There are five parameters to define in each IKE policy, as presented in Table 7-2. Table 7-2 shows the relative strength of each parameter. You can select specific values for each ISAKMP parameter using the ISAKMP standard.

Table 7-2 ISAKMP Parameters




Message encryption algorithm



Message integrity (hash) algorithm



Peer authentication method


RSA Encryption

RSA Signature

Key exchange parameters (Diffie-Hellman group identifier)

D-H Group 1

D-H Group 2

ISAKMP-established SA's lifetime

86,400 seconds

Less than 86,400 seconds

You should determine IKE policy details for each IPSec peer before configuring IKE. Figure 7-2 shows a summary of some IKE policy details that will be configured in examples in this chapter. The authentication method of RSA signatures is used for CA support.


Refer to Chapter 6 for a detailed explanation of Steps 3 through 6.

3. PIX CA Support Overview | Next Section Previous Section

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