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Topic # 138657 12-Jan-2014 17:50 Send private message

I have a TP-LINK TD-W8980 w. firmware 0.6.0 1.3 v000e.0 Build 131012 Rel.51720n connected to Telecom NZ ADSL2+. The TP-LINK TD-W8980 has Telepermit PTC 273/13/020.  (I do understand existence of a Telepermit confirms only suitability to connect to the Telecom network and does not confirm all protocol features except it does imply basic functionality in regard to connection.)

The currently recommended MTU to connect to Telecom ADSL2+ is 1500. The TD-W8980 will not save an MTU value for PPPoA greater than 1492. I understand that with MSS-clamping, this issue may not affect TCP, but may negatively impact other protocols such as UDP.

Mercutio reported a similar issue with the TD-W8970, and earlier with the TD-W8960; Mercutio reported that TP-LINK did fix the TD-W8960.

I have reported the issue to TP-LINK and their reply includes: "... I confirmed with our R&D person, they claimed that apart from the first 8 PPP digits data, 1492 is the largest MTU we can set. And it is not likely to be able to set it to the value of 1500 in the next firmware ..."

So is TP-LINK implying that the "effective" MTU is 1492+8, ie 1500?

Whatever, it seems one should be reserved about choosing a TP-LINK TD-W8980 to connect to Telecom ADSL2+ until TP-LINK fixes the firmware to allow the Telecom NZ recommended PPPoA MTU to be set. It seems to be a matter that TP-LINK should have been on top of since at least their TD-W8960 model.

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  Reply # 966112 13-Jan-2014 12:49 Send private message

Are you having a actual problem with the MTU at 1492?

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  Reply # 966128 13-Jan-2014 13:12

<So is TP-LINK implying that the "effective" MTU is 1492+8, ie 1500? >

I would expect the answer is yes.

You can check if you have any real problem using this info:
http://www.tp-link.com/en/article/?faqid=190

In fact its a nice little bit of tuning.... 




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  Reply # 966282 13-Jan-2014 16:04 Send private message

robjg63: 

You can check if you have any real problem using this info:
http://www.tp-link.com/en/article/?faqid=190

In fact its a nice little bit of tuning.... 


You might find an ideal MTU for one route to one site but the internet isn't just one route to one site...

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  Reply # 966313 13-Jan-2014 17:06

Dont all the packets come via your ISP?
Wouldnt be the first time I was wrong....but I thought the MTU settings were between you and your ISP.




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  Reply # 966896 14-Jan-2014 15:37 Send private message

robjg63: Dont all the packets come via your ISP?
Wouldnt be the first time I was wrong....but I thought the MTU settings were between you and your ISP.


Your modem talks to the dslam/isam via ATM, then PPPoA (or PPPoE) works over the top of that connection, then the backhaul from the dslam/isam to your ISP's BRAS is likely Ethernet... it's layer upon layer.

From your ISP onwards every hop in the route to somewhere (eg: a website) also has a MTU limit/size. MTU path discovery is supposed to work out the minimum in the route for each connection but ICMP is often dropped or ignored by routers these days.

Long story short in practice you shouldn't see a noticeable difference between 1492 set in your modem/router because it's less than the Ethernet limit of 1500.










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