Question about NetGear DG814 DSL Modem Internet Gateway Router

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Needs to know more about MTU size changes

Hi,
I want to know more about MTU size changing. Actually I want to fragment the packets into minimium size of MTU. So I set the MTU size as 10 to 20. How come I will know that my MTU size changes are reflecting.

Thanks,
Nirmala

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Changing the MTU size to any "small" value is a bad idea -- compare it to running to the corner store every time that you need to take anything (milk, ice-cube) out of your refrigerator -- the "time" to go back-and-forth to the store (or the Internet) will be excessive, and "slow" down your productivity.

Posted on May 31, 2009

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What is MTU? wut does it stands for and what is it for?


Hello - MTU stands for Maximum Transmission Unit which is a measure of the size of a data "chunk" that can be sent over a network at a time. For example, if you are connected to the Internet using Ethernet the standard MTU is 1500 bytes while being connected over a WiFi connection would mean your using an MTU of 2272 bytes while an old fashioned modem would send 576 bytes at a time.


The MTU usually refers to the TCP/IP protocol which was developed for and is the networking standard for the Internet. MTU has standards for different networking devices, Ethernet, Modems, etc. but they can be modified for the purpose for special uses to achieve greater efficiency.


To read more about MTU there is a more complete description of it at the Wikipedia entry HERE.


Happy Networking


Mikeywaf

Mar 19, 2011 | Edimax BR-6104K Router (br6104k)

1 Answer

My status light just blips and i am unable to browse the internet


With some older Cable modems there may be speed negotiation problems, you will see the WAN LED on the router flashing really fast as soon as you connect the Cable Modem to it.

To fix the problem you need to turn both the modem and the router off (unplug the power cable). Then turn the modem on first. After that turn the router on. After that the two devices will autonegotiate the link correctly.

Note: If you see WAN light flashing rapidly and one of the LAN lights flashing fast as well, such behaviour is due to network traffic going in/out the router. Exessive traffic may be related to virus or trojan infection on one of the computers on your LAN.

You might try lowering the MTU value on your router. The MTU can be changed to accommodate fragmentation of packets; specially encoded packets with secured data can be too large for the current MTU and cause too much fragmentation.

Or:

Step 1 Open your browser, enter the IP address of your router (192.168.0.1) and click OK.

Step 2 Enter your username (admin) and leave the password field blank, which is the default password. Click OK to enter the web configuration page for the device.

Step 3 Click on the Home tab and click on WAN on the left side.

Step 4 The default MTU rate is 1500, to change this enter the number in the MTU field and click the Apply button to save your settings. (Start off with 1452)

Step 5 Test your ability to connect to those specific sites you're having trouble with. If changing the MTU rate to 1452 does not resolve the problem, start lowering it in increments of ten until the problem is resolved (you may need to go as fairly low depending on your DSL connection).

If you do not have this option, please upgrade your firmware at http://support.dlink.com/downloads

Nov 05, 2010 | D-Link AirPlus DI-524 Wireless Router

1 Answer

I'm getting twice the download speed from my Xavi 7868r router than I get from my new zyxel p660hw-d1 router. The adsl ppp led blinks green then goes to permanent orange is this an alarm? The router came...


My Zyxel cruises along at 10 megabits with no problems. If you are using PPPoE then the light will turn green as the DSL synchs up and connects, and then, after the PPP connection is made (which means that the userid and password are accepted and the device is assigned things by the connection, the light will switch to orange. It blinks when data is transmitted, so this is a NORMAL state for someone who uses PPPoE or PPPoA for their connection. For someone like me who uses ethernet encapsulation, it stays green.
Are you actually on Telefonica de Espana? One of the things I've noted on my pre-configured router is that it uses specific settings for the ATM stuff that matches what my local Telco expects, and I can't change them. I would not expect this to work with anyone else's system.
But 50% speed? Wow, first thing I'd check is my MTU. Depending on what system you have, I'd set my max MTU to about 1400 on my PC to see if that fixes anything. The issue might be that you are getting fragmenting in the connection between the ZyXel and the main system. Problem is that most MTU (Maximum Transmission Unit) sizes are set to 1500. But the PPPoE adds an extra header for the encapsulation, which leaves you with a smaller MTU on that link, around 1492 I think so 1400 is a good number. Systems handle this 2 ways, One is to set the "don't fragment" bit, which means that an ICMP message is sent back, and the sending system has to learn that and start sending smaller packets - this takes time and can effect a short speed test.
The other possibility is that the Don't Fragment bit is not set. Then the router has to actually manipulate the packets - it splits them into multiple packets, called fragments, and the fragments then make their own way through to the final receiving system. This can be slower than just telling the sender to send smaller packets, because that only has to happen once.
It might be that while the ZyXel is fast at packets it is miserable at fragmentation because it is rarely used these days - almost everyone sets "don't fragment" and transmits smaller packets.
But there is a final thing. The ZyXel might just not be seeing the link as being as fast as your Xavi did, even though it is the same link.
You can access the ZyXel via the web and it will tell you how fast it thinks the link is. Say you are paying for a 5 meg link from your phone company. The Xavi sees it as a 5 meg link, and it has no problem getting a maxiumum of about 500 kilobits through it on a download. I have two links that are rated the same. One is seen as a 11 megabit link, the other is seen at about 6500 kilobits. I have a service call in with the telco.
Now, the ZyXel, when it is first turned on, tests the line. There are a lot of subcarriers, and it checks each for noise and loss - if it decides that most of them are not usable, it will rate the line as slower. Not sure why, it could be that the circuitry is marginal, or it could be an issue with the link or the card at the telco end.
So it decides that the link is only a 2.4 megabit link.
I would attach the Xavi to the line, attach to it with web and ask it how fast it thinks the line is.
Then I'd attach the ZyXel and see if it gets the same number. If there is a big discrepancy, I'd return it to where I got it, or if it is the telco I'd report it, they can access it remotely and see what is going on,

Jan 09, 2010 | Zyxel P660HW-D1 Router

2 Answers

My downloads stop usually after a few seconds i seem to only have this problem with about 50% of sites that i try to download from. Sometimes it happens on youtube or some sites like rapidshare. I will...


Sounds like a MTU issue. The "MTU" also means "Maximum Transmission Unit". This has to do with the largest packet that you can send on a particular link. In the old days, routers would break a large packet into fragments when they were about to transmit a packet onto a link with a lower MTU.
But this slows down the router and your connection. So these days, windows sets the "Don't Fragment" flag - and rather than break the packet, the router will respond with an ICMP packet that tells your system to send smaller packets - and your system will send smaller packets and avoid the fragmentation load and slowdown.
Problem is that sometimes the ICMp responses do not get through. Security professionals used to block all ICMP, willy-nilly. ICMP (except perhaps Ping) should never be blocked.
http://www.netheaven.com/pmtu.html is a great discussion of this.
What happens is,
1. You open a connection to site X. You tell them that the largest packet you will take is 1500 because that is the MTU on your wireless net.
2. The packets that are used to open the connection are small packets. Then, when the data starts flowing, large packets are sent. One of the packets is too big. The router between them and you sends the "whoops, too big" packet, and they filter it at their end.
3. They resend the packet and it never gets through. You eventually give up.
The problem is on their end - but let me ask this:
If you connect to the modem via a wire, does it change anything? I honestly don't think it is your problem, but it could be - if this is not the issue.
To diagnose, use this scheme to set your MTU to 576.
http://www.windowsreference.com/windows-2000/how-to-manually-set-the-mtu-size-in-windows-xp-2003-2000-vista/
and then see if you can access these sites. If you can access these sites now, then this is your problem.
I do not know anything about your topology, but if you happen to have a link to you with a reduced MTU (are you running PPPoE? Who is your supplier?) and you can figure out what that MTU is, then by setting the MTU on the wireless link to that number, you will avoid having these sites send you large packets. The ICMP response never gets there but it is never needed.
What I don't get is why a reload would EVER work. The response should either go through or not. Resending the same stream should cause a problem in the same place.
Here is an alternate theory:
Years ago, I had a problem downloading a particular file via DSL. Just one file...out of many hundreds I'd downloaded. And occasionally I had a problem with a bit of web content but who cares? But this was a file and it was a hard failure so I could look at it further.
I had my friend compress the file (which changes the pattern of data in the file), and got a copy of the compressed file. Then I blew up the file and looked in the file with a hex editor and, paid special attention to the hunk that was just past the part where the file had hung.
Well, the ping command on Linux allowed you to send a ping with not only a particular buffer length but a particular payload, and what I discovered was that when I sent a particular packet (that matched the part of the file that failed) I would never get the response.
I called my ISP and explained that I had a ping that would fail when certain data came through but not when other data came through and that I could ping with a packet of a certain length but when I sent a packet with different data and the same length I never got a response.
The only difference was the data part of the packet.
I ended up at the highest tech level and they started tracing things while I ran my ping - and suddenly my link went completely dark for a minute and then started working. They had replaced my link card - on my ADSL link. It was sensitive to the content of the data. The internal indication they got was that the checksum had failed so the packet was dropped, but the hardware was broken and they were miscalculating the checksum, the packet was actually fine when they looked at the dump. (As I recall, 64 bytes of hex 0f0f0f0f0f0f would fail so it was simple to see that what I sent got there, by inspection).
Now, there is a possibility that the link, either the wi-fi at your end or the DSL link hardware is broken and is broken in a way (like mine was) where it is data sensitive. And some data causes it to be likely to toss the packet on the floor, while other data makes it through just fine.
If I was a techie, I would get a copy of a program called Wireshark. This program used to be called ethereal, and it allows you to trace what goes over your IP connection and through your adapter. If you understand IP you might be able to see where the TCP connection stalls and then when you restart it and it goes through, what is in the packet following the stall. If it fails in the same way every time, it might be possible to use something like wget for windows (which see: like http://gnuwin32.sourceforge.net/packages/wget.htm ) and to get a particular file and cause a reliable failure, or a high percentage of failures, and with that in hand to call your ISP.
If you can cause the problem to happen with multiple computers at your end you might be able to get them not to look at your computer.
A trace of a failing wget would be the next thing I would look for. Hope this helps, let us know what happens.
A final theory is that, to stop people from filesharing, the ISP is rolling your IP address every few minutes. By breaking a PPPoE connection and forcing the router to reconnect and always supplying a new IP address, or doing the same sort of thing with a short DHCP timeout and rolling the IP address, they could do a lot to interfere with the popular programs that share large files, like bittorrent and such.
If you happened to be downloading a file when your IP address changed, it would, perforce, cause your download to be interrupted...the ISP could stop sending packets with your old IP address instantly, and, in this case, control-r would have a good chance of working, since well designed web programs use cookies and magic URLs to identify the stream, and not origin IP since so many people mash a bunch of identities into different streams from one IP address.
You could diagnose this by connecting to your router and determining what your exterior IP address is, then, when you get a hang, see if the exterior IP address has changed.

Jan 08, 2010 | Linksys Wireless-G WRT54G Router

1 Answer

Router


You need to know how to change the MTU packet size on the XBox?

Jul 10, 2008 | Compaq (CISCO7204VXR-DC-RF) Router

1 Answer

Slow internet responses


MTU Means Maximum transmission unit In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-to-point serial links). A higher MTU brings higher bandwidth efficiency. However, large packets can block up a slow interface for some time, increasing the lag for further packets. For example, a 1500 byte packet, the largest allowed by Ethernet at the network layer (and hence most of the Internet), would tie up a 14.4k modem for about one second

Feb 26, 2008 | D-Link DI 514 Wireless Router (DI-514)

1 Answer

MTU


MTU, Partial Loss of Internet Connection, and Performance MTU (Maximum Transmission Unit) is the largest packet a network device transmits. The best MTU setting for NETGEAR equipment is often just the default value. MTU is sometimes presented as something that can be easily changed to improve performance, but in practice this may cause problems. Leave MTU unchanged unless one of these situations applies: 1. You have problems connecting to your ISP or other Internet service, and their technical support (or NETGEAR's) recommends changing MTU. For example, these services may require an MTU change: * Yahoo email * MSN * AOL's DSL service 2. You use VPN, and have severe performance problems. 3. You used a program to "optimize" MTU for performance reasons, and now you have connectivity or performance problems. * An easy solution to most problems is to change MTU to 1400. * If you are willing to experiment, gradually reduce MTU as described in "Details of MTU", below. How to Change a Computer's MTU Size Note: If you change MTU on one computer, it is likely you will want to change it on your other computers, switches, and routers, as well. Instructions for changing MTU on other NETGEAR devices is found in the Reference Manuals. The third party Dr. TCP software can be used to change the MTU setting. 1. Download it from this link, choosing the zip file or the exe file at the top of the page. 2. Run the utility. 3. In the Adapter Settings pull down, select the Ethernet driver and adapter used to connect with the network. 4. In the MTU box, type the MTU size you are trying. 5. Click in any other box, without changing the data there. 6. Click Save. 7. Click Exit. 8. Restart the computer. Details About MTU A packet sent to a device larger than its MTU is broken into pieces. Ideally, MTU would be set to the same ? large ? value on all your computers, routers and switches, as well as on all the parts of the Internet that you access. But you cannot control the MTU on the Internet, and in practice the optimum MTU size on your LAN is related to your hardware, software, wireless interference, etc. * Tweaking MTU size may work well in one situation, but cause performance and connection problems in others. * When network devices with different MTU settings communicate, packets are fragmented to accommodate the one with the smallest MTU. * Windows XP sets MTU automatically, that is, it optimizes computer MTU for you. This Microsoft article explains resolving lack of connection to a broadband ISP using Windows XP: http://www.microsoft.com/windowsxp/using/networking/learnmore/ppoe.mspx * Once a network device fragments a packet, the data stays fragmented until arriving at the destination computer. Setting MTU size is a process of trial-and-error: start with the maximum value of 1500, then reduce the size until the problem goes away. Using one of these values is likely to solve problems caused by MTU size: * 1500. The largest Ethernet packet size; it is also the default value. This is the typical setting for non-PPPoE, non-VPN connections. The default value for NETGEAR routers, adapters and switches. * 1492. The size PPPoE prefers. * 1472. Maximum size to use for pinging. (Bigger packets are fragmented.) * 1468. The size DHCP prefers. * 1460. Usable by AOL if you don't have large email attachments, etc. * 1430. The size VPN and PPTP prefer. * 1400. Maximum size for AOL DSL. * 576. Typical value to connect to dial-up ISPs.

Feb 19, 2006 | NetGear DG814 DSL Modem Internet Gateway...

1 Answer

Connection problems


The current firmware allows the maximum transmission unit to be changed if needed. If you are having a problem sending or receiving email, or connecting to secure sites such as eBay, banking sites, and Hotmail, we suggest lowering the MTU in increments of ten (Ex. 1492, 1482, 1472, etc). What is my MTU? - To find the proper MTU Size, you?ll have to do a special ping of the destination you?re trying to go to. A destination could be another computer, or a URL. Step 1 Click on Start and then click Run. Step 2 Windows 95, 98, and ME users type in command (Windows NT, 2000, and XP users type in cmd) and press Enter (or click OK). Step 3 Once the window opens, you?ll need to do a special ping. Use the following syntax: ping [url] [-f] [-l] [MTU value] Example: ping yahoo.com -f -l 1472 You should start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 2 until you get a fragmented packet. Take that value and add 28 to the value to account for the various TCP/IP headers. For example, lets say that 1452 was the proper value, the actual MTU size would be 1480, which is the optimum for the network we?re working with (1452+28=1480). Once you find your MTU, you can now configure your router with the proper MTU size. Note: AOL DSL+ users must use MTU of 1400. To change the MTU rate follow the steps below: For the DI-514, DI-524, DI-604, DI-614+, DI-624, DI-754, DI-764, DI-774, and DI-784: Step 1 Open your browser, enter the IP address of your router (192.168.0.1) and click OK. Step 2 Enter your username (admin) and password (blank by default). Click OK to enter the web configuration page for the device. Step 3 Click on the Home tab and go to the WAN tab. Step 4 The default MTU is 1500, to change this enter the number in the MTU field and click the Apply button to save your settings. Step 5 Test your email. If changing the MTU does not resolve the problem, continue changing the MTU in increments of ten. Note: If you have the European version of the DI-604, this procedure will not work. If you do not have an MTU option, please upgrade your firmware.

Feb 16, 2006 | D-Link Air Xpert DI-774 Wireless Router

1 Answer

Connection problems


The current firmware allows the maximum transmission unit to be changed if needed. If you are having a problem sending or receiving email, or connecting to secure sites such as eBay, banking sites, and Hotmail, we suggest lowering the MTU in increments of ten (Ex. 1492, 1482, 1472, etc). What is my MTU? - To find the proper MTU Size, you?ll have to do a special ping of the destination you?re trying to go to. A destination could be another computer, or a URL. Step 1 Click on Start and then click Run. Step 2 Windows 95, 98, and ME users type in command (Windows NT, 2000, and XP users type in cmd) and press Enter (or click OK). Step 3 Once the window opens, you?ll need to do a special ping. Use the following syntax: ping [url] [-f] [-l] [MTU value] Example: ping yahoo.com -f -l 1472 You should start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 2 until you get a fragmented packet. Take that value and add 28 to the value to account for the various TCP/IP headers. For example, lets say that 1452 was the proper value, the actual MTU size would be 1480, which is the optimum for the network we?re working with (1452+28=1480). Once you find your MTU, you can now configure your router with the proper MTU size. Note: AOL DSL+ users must use MTU of 1400. To change the MTU rate follow the steps below: For the DI-514, DI-524, DI-604, DI-614+, DI-624, DI-754, DI-764, DI-774, and DI-784: Step 1 Open your browser, enter the IP address of your router (192.168.0.1) and click OK. Step 2 Enter your username (admin) and password (blank by default). Click OK to enter the web configuration page for the device. Step 3 Click on the Home tab and go to the WAN tab. Step 4 The default MTU is 1500, to change this enter the number in the MTU field and click the Apply button to save your settings. Step 5 Test your email. If changing the MTU does not resolve the problem, continue changing the MTU in increments of ten. Note: If you have the European version of the DI-604, this procedure will not work. If you do not have an MTU option, please upgrade your firmware.

Feb 16, 2006 | D-Link Air Xpert DI-774 (DWL-774) Wireless...

1 Answer

Connection problems


The current firmware allows the maximum transmission unit to be changed if needed. If you are having a problem sending or receiving email, or connecting to secure sites such as eBay, banking sites, and Hotmail, we suggest lowering the MTU in increments of ten (Ex. 1492, 1482, 1472, etc). What is my MTU? - To find the proper MTU Size, you?ll have to do a special ping of the destination you?re trying to go to. A destination could be another computer, or a URL. Step 1 Click on Start and then click Run. Step 2 Windows 95, 98, and ME users type in command (Windows NT, 2000, and XP users type in cmd) and press Enter (or click OK). Step 3 Once the window opens, you?ll need to do a special ping. Use the following syntax: ping [url] [-f] [-l] [MTU value] Example: ping yahoo.com -f -l 1472 You should start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 2 until you get a fragmented packet. Take that value and add 28 to the value to account for the various TCP/IP headers. For example, lets say that 1452 was the proper value, the actual MTU size would be 1480, which is the optimum for the network we?re working with (1452+28=1480). Once you find your MTU, you can now configure your router with the proper MTU size. Note: AOL DSL+ users must use MTU of 1400. To change the MTU rate follow the steps below: For the DI-514, DI-524, DI-604, DI-614+, DI-624, DI-754, DI-764, DI-774, and DI-784: Step 1 Open your browser, enter the IP address of your router (192.168.0.1) and click OK. Step 2 Enter your username (admin) and password (blank by default). Click OK to enter the web configuration page for the device. Step 3 Click on the Home tab and go to the WAN tab. Step 4 The default MTU is 1500, to change this enter the number in the MTU field and click the Apply button to save your settings. Step 5 Test your email. If changing the MTU does not resolve the problem, continue changing the MTU in increments of ten. Note: If you have the European version of the DI-604, this procedure will not work. If you do not have an MTU option, please upgrade your firmware.

Feb 16, 2006 | D-Link AirPro DI-764 Wireless Router

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