Core issue
The problem occurs due to the presence of Cisco bug ID CSCsd97167.
This issue occurs when a PIX firewall is configured as a Point-to-Point Protocol over Ethernet (PPPoE) client.
Resolution
For a workaround, refer to the Software Download page and upgrade the PIX Firewall software to version 6.3(5.123).
Note: If version 6.3(5.123) is not available, upgrade to version 6.3.5.GD or later.