[cisco-nas] ATM/L2TP/PPP MTU question

[Dennis Peng]

Tassos Chatzithomaoglou [achatz at forthnet.gr] wrote:
> 
> Here is the network diagram:
> 
>           DSL         ATM              ATM         ATM       ETH
> ADSL CPE ----- DSLAM ----- ATM SWITCH ----- BBRAS ----- LNS ----- INTERNET
>     |-------------- PPPoE/A------------------||----L2TP---|
>     |----------------------- PPP -------------------------|
> 
> What is the recommended MTU setting for the ip/atm connection between 
> BBRAS/LNS (default for ATM is 4470), if we have to :
> 
> 1. Do not change anything on user's side
> 2. Do not load BBRAS/LNS with extra fragmentation/reassembly
> 
> Should i make it 1540 (1500 + 40 bytes of IP, UDP and L2TP headers) ?

Yes, 1540 should do it if you are going Cisco to Cisco. If you are
going inter-vendor, it depends on the L2TP header options the other
vendor may use (eg. data sequencing would add additional
overhead). You may want to increase to 1550 to be safe.

Dennis

> -- 
> ***********************************
>    Chatzithomaoglou Anastasios
> Network Design & Operations Center
>           FORTHnet S.A.
>       <achatz at forthnet.gr>
> ***********************************
> 
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> cisco-nas at puck.nether.net
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-- 
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      ||        ||                                 Dennis Peng
      ||        ||        Cisco Systems, Inc.      Escalation Engineer
     ||||      ||||       170 West Tasman Drive    Phone: (408) 526-6143
 ..:||||||:..:||||||:..   San Jose, CA 95134       Fax:   (408) 232-2343
   Cisco Systems Inc.                              dpeng at cisco.com
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