Cisco Cisco IOS Software Release 12.2(27)SBC

To enable Layer 2 Transport Protocol (L2TP) congestion avoidance, use the l2tp congestion-control 
command in global configuration mode. To disable L2TP congestion avoidance (default state), use the 
no form of this command.

This command has no arguments or keywords.

L2TP congestion avoidance is disabled.

Global configuration

The l2tp congestion-control command operates as a user-controlled on-off switch. An L2TP sliding 
window mechanism is enabled or disabled by this command, but only for those tunnels that come up after 
the configuration has been applied. In other words, tunnels that exist when the l2tp congestion-control 
command is enabled remain unaffected by the command. The reason for this is to avoid a situation where 
the the sliding window mechanism is enabled at a point in transmissions where the existing size of the 
resend queue is much larger than the congestion window. It is not desirable, nor is there a reason, for the 
configuration to have to apply to all L2TP tunnels. 

The congestion window size is not allowed to exceed the size of the advertised window obtained from 
the receive window size set by the l2tp tunnel receive-window VPDN group configuration command. 
Lowering the value of the receive window will result in lowering the number of calls per second being 
negotiated, and if a network is congested, the receive window size should be lowered. Increasing this 
value depends on how congested the network is. When the network becomes less congested, the receive 
window size can be increased again.

The following example enables L2TP congestion avoidance:

Router(config)# l2tp congestion-control

12.2(28)SB

This command was introduced.

12.4(15)T

This command was integrated into Cisco IOS Release 12.4(15)T and support 
was added for L2TP congestion avoidance statistics.

Specifies the size of the advertised receive window.