FYI: Recommended MTU on Coms Network

In case anyone was wondering (I recall one post asking this), I raised a ticket this morning asking what the recommended MTU was on the Coms network. Got a reply within 3 hours (IMHO pretty good for an RFI; they normally go to the bottom of the pile!) stating 1492. Ping fragmentation tests seems to confirm this value so have updated my router accordingly and everything seems to be nipping along quite nicely.

Mine is 1458, i thought 1492 was for PPPoE connections, where as we're on PPPoA

Well, we're defo on PPPoA but if you do a ping fragmentation test, 1464 pings correctly and 1465 fragments. When you add the +28 for the headers, that makes 1492...which is what Coms Tech Support recommended

Yes, 1492 is for PPPoE connections, it's 1500 0n BTw PPPoA connections.

Ive always been on 1458 its what my router defaults too, what advantages (if any) would i get from changing it to 1500?

In reply to a post by SimpleSimon1:

1465 fragments.

Are you sure you've freed MTU to 1500 everywhere , in router and PC OS, before doing ping fragmentation test?

Who supplies Coms connection? If BTw, ping 1472 should not fragment.

Fewer fragmented packets?

Ah, fair point. I'm set to 1500 in the OS but 1496 in the Router BUT my router is a 1-year old TP-Link TD-W8960N Wireless N ADSL2+ jobbie and according to both screen legend and doco, only supports a max MTU setting of 1492!

I wonder if this is the rule rather than the exception as far as routers go and that's why they're suggesting a value of 1492 rather than 1500?

Fair point! If your router limits you then your hands are tied and you just pay by the odd 8 byte unnecessary fragmentation.

Duuno how general this is but my ISP's Orange/EE Brightbox was limited also to MTU <= 1492 until an update & our nagging got them to change it in late 2012 & now we're home free.

This may help some -

http://www.tp-link.pt/article/?id=69

http://www.tp-link.com/en/article/?faqid=190

Is there anywhere on Coms site that states what settings we should use for our modems?

Edited by deleted (Tue 25-Feb-14 05:17:19)

Thanks for those. The second link is how I carried out the fragmentation test. The first link is interesting cos the screenshots (albeit for a different model of TP-Link router) show a default MTU of 1500, rather than 1480 (the default for my TP-Link router). Maybe a firmware update would change this but TBH, performance is more than acceptable with an MTU of 1492 so I'm not particularly worried. Also

"Is there anywhere on Coms site that states what settings we should use for our modems?"

As mentioned above, I originally raised a ticket directly with Coms support and got a reply from a "senior network analyst" stating that they recommend a value of 1492. So that's what I'm using.

I use 1500 mtu on my speedtouch 585 v6

1500 (1472 + 28 for header) checks out OK here. Maybe 1492 is being suggested to allow an 8 byte safety margin.

In reply to a post by My_brain_hurts:

Is there anywhere on Coms site that states what settings we should use for our modems?

I dunno! I was the one telling you Commies the correct values and how to find it.

There's an important omission from your 2nd link. Before carrying out said tests you must ensure that MTU is not restricted to under 1500 anywhere in your system, router, PC, or OS.

Edited by XRaySpeX (Tue 25-Feb-14 13:08:21)

Sorry, my bad. I was asking about the list of modem settings not just MTU. I've started a new thread on it as I think maybe there are a few peeps out there who could use the gen to check their modems are correctly set up.