[cisco-bba] LAC/LNS --> L2TP MTU issues

Christian Schmit cschmit at vo.lu
Mon May 15 16:57:32 EDT 2006

Recently we encountered MTU issues on our
DSL connections. We run a typical LAC-LNS 
setup with the telco. LAC on the telcos 
side are ERX boxes while the LNS used are
Cisco7200 with G1.

Affected by the MTU problems were some IPSEC VPNs
and radius connections.

While browsing through different sample configs
on the Internet I found the following commands
to be useful to address the mtu problem.

virtual template commands:

- ip tcp adjust-mss xxxx
- ppp mtu adaptive
- ip mtu xxxx
- mtu xxxx
- clearing the DF bit using a route-map

vpdn-group commands:

- ip mtu adjust
- ip pmtu

My current LNS config regarding MTU is:

vpdn-group 1
  protocol l2tp
  virtual-template 1
 terminate-from hostname LAC
 lcp renegotiation always
 l2tp tunnel password 7 xxxxxxxxxxxxxxxxxxxxxxxx
 ip mtu adjust
interface Virtual-Template1
 ip unnumbered Loopback1
 ip tcp adjust-mss 1420
 ip mroute-cache
 peer default ip address pool DSL_POOL1
 ppp authentication pap chap

If anybody could comment on what combination of
commands is best to be used to solve the mtu 
problem would be great.


More information about the cisco-bba mailing list