Core issue
The issue is the maximum length for the serial failover cable.
Resolution
The serial failover cable on the PIX Firewall is limited to six feet. This is the only size serial failover cable that Cisco makes. As such, this is the only length supported.
However, the pinouts for the serial failover cable are provided in the Failover Cable Pinouts section of Cable Pinouts.
With the introduction of LAN based failover in PIX Firewall software version 6.2, any unused LAN interface can be used as the failover link and there is no limit on length.
The only requirement is that the two PIX Firewalls have Layer 2 (L2) connectivity.
For more information, refer to the Configuring LAN-Based Failover section of Using PIX Firewall Failover.
