NAS Configuration
Router(config)# vpdn enable
!
Router(config)# vpdn-group 1
Router(config-vpdn)# request-dialin
Router(config-vpdn-req-in)# protocol l2tp
Router(config-vpdn-req-in)# domain cisco.com
!
Router(config-vpdn)# initiate-to ip 172.22.66.25
Router(config-vpdn)# local name NAS1
!
Router(config)# l2tp congestion-control
The following example shows L2TP tunnel activity, including the information that L2TP congestion control is enabled. Note that the slow start threshold is set to the same size as the remote receive window size. The Remote RWS value advertised by the remote peer is shown in the Remote RWS field. When the actual RWS value differs from the advertised value, the actual RWS value will be displayed as
In Use Remote RWS <value>.
Router# show vpdn tunnel l2tp all
L2TP Tunnel Information Total tunnels 1 sessions 1
Tunnel id 30597 is up, remote id is 45078, 1 active sessions
Tunnel state is established, time since change 00:08:27
Tunnel transport is UDP (17)
Remote tunnel name is LAC1
Internet Address 172.18.184.230, port 1701
Local tunnel name is LNS1
Internet Address 172.18.184.231, port 1701
Tunnel domain unknown
VPDN group for tunnel is 1
L2TP class for tunnel is
4 packets sent, 3 received
194 bytes sent, 42 received
Last clearing of "show vpdn" counters never
Control Ns 2, Nr 4
Local RWS 1024 (default), Remote RWS 256
In Use Remote RWS 15
Control channel Congestion Control is enabled
Congestion Window size, Cwnd 3
Slow Start threshold, Ssthresh 256
Mode of operation is Slow Start
Tunnel PMTU checking disabled
Retransmission time 1, max 2 seconds
Unsent queuesize 0, max 0
Resend queuesize 0, max 1
Total resends 0, ZLB ACKs sent 2
Current nosession queue check 0 of 5
Retransmit time distribution: 0 0 0 0 0 0 0 0 0
Sessions disconnected due to lack of resources 0
Control message authentication is disabled
The following partial output from the
debug
vpdn
l2x-events command shows that congestion occurred. The congestion window size and the slow start threshold have been reset due to a packet retransmission event.
Router# debug vpdn l2x-events
!
*Jul 15 19:02:57.963: Tnl 47100 L2TP: Congestion Control event received is retransmission
*Jul 15 19:02:57.963: Tnl 47100 L2TP: Congestion Window size, Cwnd 1
*Jul 15 19:02:57.963: Tnl 47100 L2TP: Slow Start threshold, Ssthresh 2
*Jul 15 19:02:57.963: Tnl 47100 L2TP: Remote Window size, 500
*Jul 15 19:02:57.963: Tnl 47100 L2TP: Control channel retransmit delay set to 4 seconds
*Jul 15 19:03:01.607: Tnl 47100 L2TP: Update ns/nr, peer ns/nr 2/5, our ns/nr 5/2
!
The following partial output from the
debug
vpdn
l2x-events command shows that traffic has been restarted with L2TP congestion avoidance operating in slow start mode.
Router# debug vpdn l2x-events
!
*Jul 15 14:45:16.123: Tnl 30597 L2TP: Control channel retransmit delay set to 2 seconds
*Jul 15 14:45:16.123: Tnl 30597 L2TP: Tunnel state change from idle to wait-ctl-reply
*Jul 15 14:45:16.131: Tnl 30597 L2TP: Congestion Control event received is positive acknowledgement
*Jul 15 14:45:16.131: Tnl 30597 L2TP: Congestion Window size, Cwnd 2
*Jul 15 14:45:16.131: Tnl 30597 L2TP: Slow Start threshold, Ssthresh 500
*Jul 15 14:45:16.131: Tnl 30597 L2TP: Remote Window size, 500
*Jul 15 14:45:16.131: Tnl 30597 L2TP: Congestion Ctrl Mode is Slow Start
!