Telecom VDSL and MTU size ...

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guardy

8 posts

Wannabe Geek

#143309 10-Apr-2014 08:37

Hi all,

I'm a little confused. I guess that could be a general statement but let me elaborate.

I have a Cisco 887VA router that I want to use for our home Internet connection. (Telecom VDSL)
I have been using the supplied router from Telecom but decided it was about time I setup the Cisco properly.

According to the Telecom website, the MTU should be set to "1500 or auto"
As such, I didn't change the mtu size in the config.
I ran some ping tests to check and it seems that it was set correctly at 1500.

However, various sites were slow and some simply timed out. As an example I could (eventually) get to www.telecom.co.nz but trying to log into "mytelecom" timed out.
Also www.speedtest.net timed out on me.

Adding the following lines into my config under the Dialer interface fixed this up:
mtu 1492
ip tcp adjust-mss 1452

It's all working so that all well and good. I'd just like to know *why* it's working. :)

I understand that typically a PPPOE connection requires a 1492 size MTU but then, why does Telecom advise in the VDSL settings to use 1500?
And when using 1500 it just doesn't work properly.

Or is my understanding of this simply ... flawed?

Thanks for any insights on this.

PeterReader

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#1022601 10-Apr-2014 08:37

Hello... Our robot found some keywords in your post, so here is an automated reply with some important things to note regarding broadband speeds.

If you are posting regarding DSL speeds please check that

- you have reset your modem and router

- your PC (or other PCs in your LAN) is not downloading large files when you are testing

- you are not being throttled by your ISP due to going over the monthly cap

- your tests are always done on an ethernet connection to the router - do not use wireless for testing

- you read this topic and follow the instructions there.

Make sure you provide information for other users to help you. If you have not already done it, please EDIT your post and add this now:

- Your ISP and plan

- Type of connection (ADSL, ADSL2, VDSL)

- Your modem DSL stats (do not worry about posting Speedtest, we need sync rate, attenuation and noise margin)

- Your general location (or street)

- If you are rural or urban

- If you know your connection is to an exchange, cabinet or conklin

- If your connection is to a ULL or wholesale service

- If you have done an isolation test as per the link above

Most of the problems with speed are likely to be related to internal wiring issues. Read this discussion to find out more about this. Your ISP is not intentionally slowing you down today (unless you are on a managed plan). Also if this is the school holidays it's likely you will notice slower than usual speed due to more users online.

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plambrechtsen

1948 posts

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#1022614 10-Apr-2014 08:40

With PPPoE as long as your router supports a MTU on the VDSL interface of 1508 then you can have a PPPoE MTU of 1500.

Have a read of this, as it should apply here in NZ: http://efemoral.lovius.net/achieving-an-mtu-of-1500-on-bt-fttc/

guardy

8 posts

Wannabe Geek

#1022694 10-Apr-2014 11:19

Thanks for your reply and link. That was very helpful.
I've since adjusted the MTU on the 887VA now to be 1500.

Just for others' reference, it's slightly different than is listed in the link (likely due to router/IOS differences).

For the 887VA it needs to be as follows:

interface Ethernet0
mtu 1508
no ip address
!
interface Ethernet0.10
encapsulation dot1Q 10
pppoe enable group global
pppoe-client dial-pool-number 1
pppoe-client ppp-max-payload 1500

The important commands being "mtu 1508" under the Ethernet0 interface and "pppoe-client ppp-max-payload 1500" under the sub-interface.

After doing a debug of ppp negotiation if shows a "correct" size of 1500 being requested and accepted.

After checking it out further, it seems that if you have the following under the Dialer interface then ppp negiation will negotiate 1492:
mtu 1492
ip tcp adjust-mss 1452

If I have neither of those options (as I originally did expecting it would negotiate 1500 automatically) the debug shows multiple attempts at trying to negotiate 1492 and 1508 but failing.
Even though the pings I've done would indicate an MTU size of 1500.

Thanks again for your help. Much appreciated. :)

plambrechtsen

1948 posts

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Inactive user

#1022739 10-Apr-2014 12:02

Glad it's sorted. I would have presumed you wouldn't set the MTU and ip tcp adjust-mss on the dialer at all and leave it alone (well that's what the second link says)
A few more links:

http://www.experts-exchange.com/Hardware/Networking_Hardware/Routers/Q_27286980.html
https://community.bt.com/t5/BT-Infinity-Speed-Connection/Infinity-on-Cisco-Router/td-p/149185/page/2

Ragnor

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#1022808 10-Apr-2014 12:56

plambrechtsen: With PPPoE as long as your router supports a MTU on the VDSL interface of 1508 then you can have a PPPoE MTU of 1500.

Have a read of this, as it should apply here in NZ: http://efemoral.lovius.net/achieving-an-mtu-of-1500-on-bt-fttc/

Does this apply to UFB also?

guardy

8 posts

Wannabe Geek

#1022860 10-Apr-2014 13:54

plambrechtsen: Glad it's sorted. I would have presumed you wouldn't set the MTU and ip tcp adjust-mss on the dialer at all and leave it alone (well that's what the second link says)
A few more links:

http://www.experts-exchange.com/Hardware/Networking_Hardware/Routers/Q_27286980.html
https://community.bt.com/t5/BT-Infinity-Speed-Connection/Infinity-on-Cisco-Router/td-p/149185/page/2

Sorry, I may not have been very clear in my explanation. I've essentially done exactly what is in that second link. Added those two lines to the Ethernet0 interface and sub-interface respectively then removed the extraneous lines from the Dialer interface.

I was simply trying different options to see what the debug ppp negotiation threw back at me. :)

guardy

8 posts

Wannabe Geek

#1022867 10-Apr-2014 13:55

Ragnor:
plambrechtsen: With PPPoE as long as your router supports a MTU on the VDSL interface of 1508 then you can have a PPPoE MTU of 1500.

Have a read of this, as it should apply here in NZ: http://efemoral.lovius.net/achieving-an-mtu-of-1500-on-bt-fttc/

Does this apply to UFB also?

Without being able to actually test it, from what I've read, yes, this would apply to UFB also.

Out of interest, what Cisco equipment are you using for UFB?