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(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.