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RE: PPPoE overhead [7:97799] posted 03/21/2005
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PPPoE adds 8 bytes of overhead.

See the following for more information on PPPoE:

http://www.cisco.com/en/US/tech/tk175/tk15/technologies_white_paper09186a0080093e55.shtml

http://www.cisco.com/en/US/tech/tk175/tk15/technologies_tech_note09186a0080093bc7.shtml


L2TP version 2 itself (typically) adds 12 bytes. But also add the IP and UDP
headers (L2TPv2 operates over IP/UDP), giving a total of 12 + 8 + 20= 40
bytes.


If you are using (compulsory mode) L2TP, remember that the following occurs
during L2TP session setup for an inbound connection from a remote access
client):

1. The LAC negotiates the Link Control Protocol (LCP), including the MRU [in
simple terms, the MTU] and other options with the remote access client.

2. During L2TP session setup, the LAC passes the negotiated LCP options
(including MRU) to the LNS.

3. The LNS can then choose either to accept those LCP options passed by the
LAC, or renegotiate LCP (including the MRU) with the remote access client.


If you do run into MTU problems when running L2TP, one method of dealing
with these is to configure the 'mtu' command under the virtual template
interface, and the 'lcp renegotiate on-mismatch/always' command under the
vpdn-group. This causes the LNS to renegotiate the MRU (MTU) with the remote
client to the value specified using the 'mtu' command, irrespective of the
MRU negotiated between the LAC and the remote client.

The above method of dealing with MTU issues with L2TP works especially well
with XP as XP obeys the negotiated MRU (unlike some OS' which don't!).

Anyway, have fun!

HTH.

Mark

CCIE#6280 / CCSI#21051 / JNCIS / etc.

Author: http://www.amazon.com/exec/obidos/ASIN/1587051044/






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