04-12-2008 02:37 AM - edited 03-05-2019 10:21 PM
Hi!
I'm trying make a lab-setup using a Cisco 1841 (IOS 12.4(4)T2) for eth-bridging over L2TP using BCP/PPP/L2TP
according to RFC 3518. The peer-end (LAC) of the connection is a Linux device running xl2tpd
together with a patched pppd supporting bcp.
The idea of the lab-setup is to get a transparent LAN to LAN connection supporting full size 1500 bytes IP
packets.
Setup is as follows.
HostA-LAN-(172.X.X.150)---(BCP/PPP/L2TP)---(172.31.11.10)-HostB-LAN
Host A(192.168.1.10): Connected To Linux Router
Host B(192.168.1.11): Connected to Cisco 1841
The setup is working fine with one exception. I can't send full size (1500B) IP packets
from Host B to Host A. Smaller packet are passed without problems, Host A can also recieve
a DHCP-lease from a DHCP server behind the Cisco 1841 without problems.
It is possible to send full size 1500b packets from HostA to HostB. I can see that those
packets are being fragmented after leaving the Linux router, and also being reassembled
correctly at Host B.
I guess my problems are caused by the fact that large packets must be fragmented before being
tunnled inside the l2tp tunnel due to the overhead added by l2tp and bcp.
In order to run BCP over L2TP I had to disable routing in the Cisco 1800. When routing
is disabled no fragmentation can be done at the Cisco end?!
The L2TP tunnel carries the PPP packets wich carries the Eth-frames. It runs over UDP over
IP and adds the following headers.
[20b IP][8b UDP][12b L2TP(incl PPP enc)][14b Eth][-------IP DATA Payload-------] = 54Bytes Overhead
I've tried all the L2TP MTU tuning hints in this document without success (probably because I have to
disable routing in order to run BCP) http://www.cisco.com/warp/public/471/l2tp_mtu_tuning.html
Is this a Cisco supported scenario or am I asking for the impossible?
if I replace the Cisco router with a Linux box everything is working fine.
See attached configuration.
BR,
Anders
04-17-2008 01:54 PM
L2TP uses the registered User Datagram Protocol (UDP) port 1701, and the whole L2TP packet is encapsulated within the UDP datagram.
As per normal UDP operation, the tunnel initiator selects an available UDP port and sends port number 1701 to the UDP destination. In the reply, the destination port number is the same as the source port number that is used in the incoming UDP header. The source port is set on the basis of any free port that is found. After the source and destination ports are established, the ports must remain the same for the duration of the tunnel. The source and destination port numbers are always set to UDP port number 1701.
Below URL which will help you :
http://www.cisco.com/en/US/tech/tk801/tk703/technologies_tech_note09186a0080204aef.shtml#t2
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800a43e9.shtml#wp1002209
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide