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receive-window

Strange VPDN-GROUP behavior

I noticed some strange behavior in a vpdn-group configuration on a Cisco 876 router. I have a router with the following vpdn-group configuration:

vpdn-group pptp-group
! Default PPTP VPDN group
description pptp vpn users
accept-dialin
protocol pptp
virtual-template 10

The configuration is working perfectly and users can dialin using a PPTP connection. Backups of the configuration are made by Kiwi CatTools. Lately I noticed that the following command l2tp tunnel receive-window 256 is added to the configuration, like displayed below:

l2tp

Cisco has the following explanation for the command:

“Use the l2tp tunnel receive-window command to set the size of the advertised control channel receive window. The receive window size controls the number of L2TP control packets that can be queued by the system for processing. Increasing the size of the control channel receive window allows the system to open PPP sessions more quickly; a smaller size is desirable on networks that cannot handle large bursts of traffic… Source

Two days later the command is gone again. I asked the network engineers if they made any changes to the configuration, but they didn’t. I looked at the configuration and tried to add the command, but I am not able to add the command.

cisco-876(config)#vpdn-group pptp-group
cisco-876(config-vpdn)#l2tp tunnel receive-windows 256
^
% Invalid input detected at ‘^’ marker.

cisco-876(config-vpdn)#

I searched a little further and the command can only be added, when the dial-in protocol is changed from pptp to l2tp. Looking at the configuration above, you can see clearly that the dial-in protocol pptp is configured and the l2tp command is added.

I cannot explain this behavior. I hope some of you can…….