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New Member

ATM/L2TP/PPP MTU question

Here is the network diagram (use fixed font to see it):

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) ?

3 REPLIES

Re: ATM/L2TP/PPP MTU question

The MTU is negotiated between the endnodes on connection setup. In your case it can never become larger than 1500 or 1492 (PPPoE) on the ethernet.

The packet sizes that traverse the network can (and will) be smaller than the MTU on your ATM link.

Therefore I think that your problem about setting the MTU is a theoretical one.

A good link for this is: www.dslreports.com

They have tools that show you how well your connection works.

Regards,

Leo

New Member

Re: ATM/L2TP/PPP MTU question

>The packet sizes that traverse the network can (and will) be smaller than the MTU on your ATM link.

That's why i'm asking. Should i leave it 4470 or make it smaller (but bigger that 1500)?

What are the disadvantages/advantages of each ?

Re: ATM/L2TP/PPP MTU question

The MTU defines the maximum value.

Frame sizes will never reach this size in your example.

There is no need to change the default setting.

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