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Author Topic: MRU / MTU of 1508 on BTW 20Cn  (Read 2448 times)

Weaver

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MRU / MTU of 1508 on BTW 20Cn
« on: July 01, 2015, 06:26:00 PM »

after wasting a load of time trying to debug the undebuggable, Andrews & Arnold tech support put an end to my misery (sort of) ,  by putting my mind right. I was trying to work out why my DLink DSL-320B modems, which support RFC 4638 (allowing 1508 byte MRU / MTU for ethernet frames on the lan) so permitting you to enjoy a 1500 byte IP MRU / MTU payload as the payload of PPPoE on the Internet, did not seem to be doing what I wanted it to do. The DLink modems showed up on my Firebrick router UI as PPPoE MTU 1492 (=1500-(6+2)).

[I'm going to just write MTU in future when I in fact mean “MRU and/or MTU”, as my fingers are getting tired]

** So the truth is it *won't* ever deliver DSL MTU 1508 (=1500+8) if you are on BTw 20CN.

** Unless you are on 21CN or FTTx, then you are stuck with MTU 1492 (=1500-8).

[In addition to which you need the right modem, modem firmware, correct config in modem and friendly LAN-side ethernet link hardware and friendly router ethernet i/f, all of which must support baby jumbo frames for RFC4638]
« Last Edit: July 01, 2015, 06:29:41 PM by Weaver »
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Weaver

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Re: MRU / MTU of 1508 on BTW 20Cn
« Reply #1 on: July 01, 2015, 06:37:18 PM »

So I believe that the only possible way of getting MTU 1500 (=1508-8) if you are on 20CN is to use a Draytek Vigor 130.

Sanity check. Because the Draytek will be set up to do PPPoA on the WAN (thank god!!),.

(not PPPoEoEoA- in truth, PPP over PPPoE over ethernet over AAL5 over ATM over DSL. Ugh)

Just praying that I can speed my test Draytek up by giving it time, prayer, massage, tweakings, appeals to statistics over a better sample, and choicest offerings to the Gods of DLM.)
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kitz

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Re: MRU / MTU of 1508 on BTW 20Cn
« Reply #2 on: July 05, 2015, 01:30:01 PM »

Quote
and choicest offerings to the Gods of DLM.

Possibly your best bet  ::)
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