Protect Your PIX - Page 3

By Cisco Press | Posted Oct 3, 2001
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The first change made to this configuration is the added nameif command for the accounting DMZ, assigning a security level of 60. The next change is that you enabled this interface with the interface command. You then assigned an IP address to the interface. Next, you configured the failover parameters.

failover Commands
The failover commands are relatively simple to use. Before discussing the commands, this section takes a few moments and discusses the requirements for a failover PIX, how the primary and secondary PIX are connected, and how the failover PIX is configured.

When purchasing a PIX, consider purchasing a failover PIX at the same time. When both are purchased together, there is a significant price reduction on the failover unit. Because the PIX is generally used as the primary device protecting your network, it usually makes sense from both service and fiscal points of view to make this a redundant system.

For a PIX to failover to another PIX after failure, both firewalls must have identical hardware and identical software versions. There is a proprietary cable made specifically for connecting between PIX Firewalls. On the back of each PIX is a port labeled failover. The cable ends are labeled primary and secondary. Once the primary PIX is configured, turn the secondary PIX's power off. Connect the cable, and restore power to the secondary PIX. After a few seconds, the secondary PIX acquires a copy of the configuration on the primary PIX. Should the primary PIX fail, the secondary PIX starts establishing connections. However, any connections that exist when the primary PIX fails are dropped and need to be reestablished. After the secondary PIX is powered on with the failover cable connected, changes should only be made to the primary PIX. One limitation of the failover system on the PIX is the length of the failover cable. The length of the cable cannot be extended, and the cable is required to be used. Therefore, you cannot use a primary PIX in one physical location and the secondary PIX in another location.

The first command used is the failover active command. This command, like all commands, should only be entered on the primary PIX. This command establishes that failover is configured and that the present PIX is the primary PIX. Using the no form of this command forces the current PIX to become the secondary PIX.

The second command shown is the failover link command. You have specified that the port used for the failover is the failover port. There is one more command used regarding failover. This command, write standby, is shown at the bottom of the configuration. The write standby command should be used after each time the configuration is changed. This causes the secondary PIX to receive a copy of the current configuration.

Understanding Failover
The failover features of the PIX are similar to those used with the Hot Standby Router Protocol (HSRP) in that the standby device remains inactive until the primary device fails. The standby device, on activation, assumes the IP and Media Access Control (MAC) address of the primary unit. Likewise, the previously active device assumes the IP and MAC addresses of the formerly standby device. Because network devices do not see any change in these addresses, no new ARP entries need to be made on the hosts using the PIX Firewall.

Starting with the PIX IOS 5.0 software release, stateful failovers are supported. Prior to this release, the PIX did not maintain a copy of the connection state in the standby unit. When the primary device failed, network traffic needed to reestablish previous connections. Stateful failovers overcome this issue by passing data about the state of connections between the primary and the standby devices within state update packets. A single packet traversing the PIX can establish a new connection state. Because each connection state changes on a per-packet basis, every packet received by the currently active device requires a state update packet to be relayed to the inactive device. Although this process works very well, there are some latency-sensitive applications that will time out before the failover process is completed. In these cases, a new session will need to be established.

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