Cisco Cisco IOS Software Release 12.2(27)SBC

© 2007 Cisco Systems, Inc. All rights reserved.

The L2TP Congestion Avoidance feature provides packet flow control and congestion avoidance by 
throttling Layer 2 Transport Protocol (L2TP) control messages as described in RFC 2661. Throttling 
L2TP control message packets prevents dropped sessions when the peer’s input buffer overflows. 

Before the introduction of the L2TP Congestion Avoidance feature, the window size used to send packets 
between the network access server (NAS) and the tunnel server was set to the value advertised by the 
peer endpoint and was never changed. Configuring the L2TP Congestion Avoidance feature allows the 
L2TP packet window to be dynamically resized using a sliding window mechanism. The window size 
grows larger when packets are delivered successfully, and is reduced when dropped packets must be 
retransmitted.

L2TP congestion avoidance is useful in networks with a relatively high rate of calls being placed by 
either tunnel endpoint. L2TP congestion avoidance is also useful on highly scalable platforms such as 
the Cisco 10000 router, which supports a large number of simultaneous sessions.

Configuration information is included in the “VPDN Tunnel Management” chapter in the

, Release 12.4T, at the following URL:

This section documents new and modified commands.