MTU on VDSL Connection

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networkn

Networkn
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#97774 20-Feb-2012 13:06

Hi There!

What MTU Size are you guys using with the VDSL connections? We have been having problems at 1500-1492 and trying to work out where the issue lies. We have a Zyxel modem behind which sits a Sonicwall TZ100.

Cheers

Napster

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#583863 20-Feb-2012 13:26

Says 1492 on my VDSL connection.

I see a lot of speed fluctuation on VDSL throughput. Not sure what exactly is causing this yet.

vespaman

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#597570 20-Mar-2012 11:20

we have a zyxel P870H in bridge mode to a Mikrotik 450 with the MTU setting @ 1480. Stable and fast.

LennonNZ

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#597597 20-Mar-2012 11:46

networkn: Hi There!

What MTU Size are you guys using with the VDSL connections? We have been having problems at 1500-1492 and trying to work out where the issue lies. We have a Zyxel modem behind which sits a Sonicwall TZ100.

Cheers

You will have to ask your ISP what MTU they support. As I presume each ISP is not 100% the same in the method they terminate it.

You can do tests via ping and setting the size of the ping and setting the no-DF bit and see what you can go up to in different parts of your network.

Thanks

eXDee

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#597878 20-Mar-2012 19:59

LennonNZ:
networkn: Hi There!

What MTU Size are you guys using with the VDSL connections? We have been having problems at 1500-1492 and trying to work out where the issue lies. We have a Zyxel modem behind which sits a Sonicwall TZ100.

Cheers

You will have to ask your ISP what MTU they support. As I presume each ISP is not 100% the same in the method they terminate it.

You can do tests via ping and setting the size of the ping and setting the no-DF bit and see what you can go up to in different parts of your network.

Thanks

Is this a reliable measure though?
On Telecom with PPPoA ADSL2+, when i ping the first hop:
ping -f -l 1473 122.59.88.1Pinging 122.59.88.1 with 1473 bytes of data:
Packet needs to be fragmented but DF set.

1472 is the highest i can go. My MTU is 1500.

sbiddle

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#597890 20-Mar-2012 20:16

MSS is 28 bytes less than the MTU using ICMP.

sbiddle

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#597897 20-Mar-2012 20:21

The MTU an ISP uses will depend entirely on their setup, there is no "correct" answer. If your ISP is using static or routed IP's then a 1500 byte MTU can be set. If they're using PPPoE then the figure will vary but would typically be 1492 or 1480 depending on setup.

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#597914 20-Mar-2012 20:39

For WVS running PPPoE it has a max MTU of 1492 (1492+8bytes PPP overhead = 1500bytes) - this is what Chorus supports for PPPoE
For WVS running IP (for example DHCP) as opposed to PPPoE you can get an MTU of 1500

If you add headers and tags etc you then get upto a max frame size of 1526

For EUBA in PPPoA mode you should be able to get 1500 as PPPoA supports 1500 by default and the DSLAM does the interworking conversion from PPPoA to PPPoE
For EUBA VLAN/PPPoE mode it would be 1492 for PPPoE and 1500 for IP as per WVS.

For BUBA - these are all PPPoA so 1500 is the max - note a modem may negotiate another value below 1500 if configured to do so.

Individual ISP's may decide to use a lower MTU than what Chorus supports.