4 Replies Latest reply: Feb 12, 2011 11:18 PM by Andrew G CCIE

# Optimal MTU settings ADSL connections

Hi All,

Sorry If im posting in the wrong place

I have been looking long and hard for a answer but have come up empty handed.

The question is what is the optimal MTU setting for an ADSL connection? And more specifically how is it calculated?

After alot of forum searching I have found that the most common optimal MTU for PPPoA and PPPoE is 1462 and 1454 respectively and that this is due to filling the ATM cells more efficiently. example.  http://whirlpool.net.au/wiki/ADSL_Theory_PPP

This doesn't make sense to me, an ATM cell is 53bytes long 5bytes are for the ATM cell header and the remain 48bytes is the payload, neither 1462 or 1454 divide equally into 48.

Using this thinking wouldn't an MTU of 1488 be more optimal (1488/48=31).

Is the website I linked in above wrong or am I missing something?

Thanks

• ###### 1. Re: Optimal MTU settings ADSL connections

Hi dear,

The default MTU is 1500 and if you expeircne any issue with default MTU ... e.g. MSN /Yahoo mail not logging in or any site login issue. You can chnage the MTU to 1496.

Regards,

http://4networkprofessionals.com

• ###### 2. Re: Optimal MTU settings ADSL connections

I understand that the default is 1500 for PPPoA and 1492 for PPPoE as it requires an additional 8byte header.

The question is about optimizing the link. Understanding why I am seeing suggestions to use and 1462 for the MTU and how this has been calculated.

• ###### 3. Re: Optimal MTU settings ADSL connections

I will try some math...  The maximum Ethernet payload MTU is 1500. Let's fill all cells with data and avoid padding. 48 * 31 = 1488. One thing you are missing is that the last cell will have 8 bytes trailer (RFC 2364), so we need to reduce our data chunk by 8 bytes 1488 - 8 = 1480. Finally, 2 bytes PPP overhead gives us 1478 MTU. That's assuming PPPoA with IP over AAL5/VC-MUX.

1478 for PPPoA/VC-MUX

1470 for PPPoA/LLC

1446 for PPPoE over ATM/LLC

Hope this makes sense.

Regards

A.

• ###### 4. Re: Optimal MTU settings ADSL connections

Thanks Andrei,

Thats Excellent I really wanted an indepth answer and that is what you have provided!

Thank you