New problem - not Activating w/ Centurylink VDSL, Zyxel C1000Z, and Netgear R7000 Nighthawk

@partymonkey- you may have future issues if you can't set the router to meet the minimum requirements needed. Checking with the mfr might be a good idea in case they have defaults that are going to cause issues later on, like SIP ALG as a default for example. VDSL can have as many issues if not more that ADSL2+ if it is not installed and/or maintained correctly.

Hi Otto, yes, good point.   Although the microcell is the only piece of equipment that had difficulty with the 1492 MTU, and everything else was and is operating correctly.  (I do use IPSEC VPN as well on something else and SIP ALG enabled without a problem).

On the other hand, changing the MTU setting is less concerning but it fixes issues that are not apparent or easy to understand for many folks.  Reducing the MTU to accommodate an ISP configuration and network equipment (your own and the ISPs) does not impact much other than throughput potentially, and actually the MTU can be optimized to your network's characeristics.   The Microcell is picky about breaking up packets, yet it has no problem at all working with 1480.  I think the listed "requirement" of 1492 is simply to point out that you can't set it to 1500 which is the default in LAN network cards under Windows, because you need to account for the 8 bytes of PPPoE, but that doesn't mean that you can't use any of the other common lower settings (1480, etc) to solve a simple but frustrating to troubleshoot problem. 

What I've observed is that many modern advanced consumer routers are starting to do away with complicated settings and automating configuration through detection and firmware, so you may not find settings like "disable packet fragmentation", etc.  Since Microcells are both used in home, soho, and other business settings, we will run into routers that are less user "tweakable" unless you apply 3rd party firmware.

So I think it's still a very valid troubleshooting step to try without a major impact.

BTW, there is a simple command in windows to identify the point at which packets start fragmenting:    ping att.com -f -l MTUsize

if you do that repeatedly (reducing by 10) until you stop seeing fragmentation messages, then you start adding 1 to the mtu size until you start seeing them again, the prior number that worked is the right size, add 28 to it for the IP overhead, and you hit the optimized size for your LAN/isp config.

And, maybe I won't need a Microcell after all one day.

Cheers, and thank you for all of your help.  The guide is a wonderful piece of work.

Thank  you. I for one would love to be able to do away with my MicoCell but with the "rules and regulations" imposed by the FCC for competing cellular carriers, dead zones and zones of minimal coverage are always going to exist. And with this competitive winner take all atmosphere, and method that a carrier can do to increase subscribers, or keep what they have, they will do. I have high hopes for the new MicroCell but only time will tell if my "suspicions" come to pass.

SIP ALG has been problematic for a lot of folks so if issues do occur, you might want to disable that if possible. In theory, it shouldn't cause any interference but proper implementation of that protocol is still an issue.

The MicroCell is still particular about it's secure VPN tunneling and anything tha can possibly interfer with that, or take resources away, is going to be a problem. Verizon has a femtocell as well and they have similar issues. However, for them, it's just easier to hobble another carrier from using their network than to try and work with that carrier on their issues.

I've seen that procedure for optimizing LAN MTU before.  I've never used it because I've never had an issue that required reducing packet size.  Good to post though.

I know this is an old post, but I am another individual who had to change their MTU size to 1480 (from 1492) in order to get their Microcell to work, so I cannot thank the posters in this post enough for that solution.  

Quick question:  what are the chances of my throughput decreasing due to this change in MTU?

Thanks!

- John

MTU (Maximum Transmission Unit) is the designated packet size in bytes for data transmission.  1500 bytes is the MTU for Ethernet.  1492 bytes is the MTU for PPPoE.  556 bytes is the typical MTU for dialup.

Changing the MTU from 1492 bytes to 1480 bytes is a slight change that I doubt you will notice as far as bandwidth. On one hand, the packet size is smaller so less data is transferred per one physical frame on the network.  However, running a smaller MTU will also decrease the time to transfer that packet.  The net difference is always slightly slower though because of increased packet header information that has to be transmitted per a given amount of packet data.

Ignoring the calculations which nobody is interested in, assuming a 1 Mbyte file size and let's say a 10 hop link over a T-1 line, the time it takes to transmit that file with a MTU of 1492 bytes would be 5.6526 seconds.  If you reduce the MTU to 1480 bytes, the time required would be 5.6529 seconds.

As you can see, a slight change in the MTU from 1492 bytes to 1480 bytes has no real world difference in bandwidth.

Generally, it's logical to assume larger packets (higher MTU) are better, because of all the following factors:

1.  network - reduce number of packet headers

2.  routers - less routing decisions

3.  clients - less protocol processing and device interrupts

However, if pure throughput is not the ultimate goal, smaller packets might be more "responsive" since they take less time to travel throughout the network.  That effect might be preferred in some applications and online gaming, at the expense of throughput.

Ultimately, packet size should be decided based on the type of the desired result, considering the underlying network as well, to avoid negative factors such as fragmentation of packets.  Still one has to realize the fact that larger packets will still transmit more useful data than smaller packets, and that there is no single "best" solution for all applications.

So to answer your question, your throughput will be theoretically decreased but I doubt you will notice any difference in your network changing the MTU from 1492 to 1480.

That completely answered my question and was extremely thorough.  Thank you so much!

- John

We aim to please!

Thanks Wingman!