Cisco IOS Software Releases 12.0 Special and Early Deployments

Field Notice: RTP over PPP Is Not Backward-Compatible

Revised June 8, 1999

Products Affected

All products running Cisco IOS® Real-Time Protocol (RTP) over Point-to-Point Protocol (PPP).

Problem Description

Cisco's implementation of RTP over PPP has been modified to follow the Internet Engineering Task Force (IETF) standard. This has resulted in the new implementation of RTP over PPP being not completely compatible with Cisco's existing implementation of the feature.


Cisco implemented a version of RTP over PPP prior to standardization of the protocol by the IETF. This previous version was to be used by a specific group of customers and displayed a warning message to that effect during the configuration stage. Cisco has become aware that, in spite of the warning, some customers have implemented that version of RTP over PPP in their network(s).

Since standardization has been completed by the IETF, Cisco has made the necessary changes to Cisco IOS to ensure that it complies with the new standard. This has caused the two implementations of RTP over PPP to have incompatibilities.

Problem Symptoms

Two Cisco routers running incompatible implementations of RTP over PPP will be able to establish a valid connection, but the RTP headers will not be compressed.


There is no workaround for this problem.

Both ends of the PPP connection need to run the same implementation of RTP over PPP.

All the following releases, and all later versions of Cisco IOS software, contain the IETF standardized implentation of RTP over PPP.

Implemented in:

  • 12.0(2.3)

  • 12.0(2.3)T

  • 12.0(2.3)S

  • 12.0(2.5)PI

  • 11.3(8.1)

  • 11.3(8.1)T

  • 11.3(8.1)NA

  • 11.3(8.1)AA

Choose one of these versions or later (as appropriate to your installation):

  • 12.0(3)

  • 12.0(3)T3

  • 12.0(3)S

  • 12.0(3)PI

  • 11.3(9)

  • 11.3(9)T1

  • 11.3(9)NA1

  • 11.3(9)AA


If you are a registered CCO user and you have logged in, you can view the bug details.



CSCdm05102 (registered customers only)

The incompatibility between the versions of RTP over PPP is first documented here.

CSCdk22031 (registered customers only)

The implementation of IETF-standardized RTP over PPP occurs in this DDTS.

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:

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