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

[Dennis Peng]

Anastassios Chatzithomaoglou [achatz at forthnet.gr] wrote:
> If i leave it on the default (4470 from our side, 8000+ from telco's side) 
> will there be any problem?

Nothing immediately comes to mind.

> What are the possibilites of such a HUGE packet? Are there any 
> disadvantages?

If it is only L2TP taffic between the BBRAS and the LNS, you shouldn't
see any packets larger than 1500 + the overhead of L2TP.

Dennis

> Dennis Peng wrote:
> >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>
> >>***********************************
> >>
> >>_______________________________________________
> >>cisco-nas mailing list
> >>cisco-nas at puck.nether.net
> >>http://puck.nether.net/mailman/listinfo/cisco-nas
> >
> >
> 
> -- 
> ***********************************
>    Chatzithomaoglou Anastasios
> Network Design & Operations Center
>           FORTHnet S.A.
>       <achatz at forthnet.gr>
> ***********************************

-- 
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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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