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Bridging using BCP over L2TP tunnel


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


Setup is as follows.


Host A( Connected To Linux Router

Host B( 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)

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.




Re: Bridging using BCP over L2TP tunnel

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 :

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