Cisco IOS Software Releases 12.1 Mainline

Field Notice: NAT Issues with Cisco IOSý Software Release 12.1(4)

September 23, 2000

Products Affected



Cisco IOS Software Release 12.1(4)


Problem Description

When a Network Address Translation (NAT) router is configured with Address Overload, any application traffic going through this NAT router with embedded address and port information may not work. This problem has been noted in Cisco IOS Software Releases 12.1(3.4) and higher.

The following applications are affected:

  • File Transfer Protocol (FTP)

  • Trivial File Transfer Protocol (TFTP)

  • NetMeeting

  • Streamworks

  • RealAudio

  • NBSS

  • VDOLive

  • VXtreme


The root cause is that the FTP ls command (see Cisco bug ID CSCds15078 below) stops responding over the NAT router.

Problem Symptoms

Sample Configuration

e0 e1
ftp-Client ----- NAT Router --------- ftp-Server in out

NAT Sample Configuration 1

int Ethernet0
ip nat inside
int Ethernet1
ip nat outside
ip nat inside source list 1 pool pool1 overload
ip nat pool pool1 netmask
access-list 1 permit

NAT Sample Configuration 2

 int ethernet0
     ip nat inside
     int ethernet1
     ip nat outside
     ip nat inside source list 1 interface Ethernet1 overload
     access-list 1 permit


The fix for this defect has been integrated into Release 12.1(4.1) and is subsequently available in Release 12.1(5). Release 12.1(4.1) is an interim release available only from the Cisco TAC.


To follow the bug ID link below and see detailed bug information, you must be a registered user and you must be logged in.




FTP ls command hangs over NAT.

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods: