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What is MTU and how to change in windows ?

March 30, 2014 Leave a comment

What is MTU ?

MTU is the Maximum Transmission Unit. This is the largest physical packet size (measured in bytes) that a network can transmit. Any messages larger than the MTU are divided (broken up) into smaller packets (fragmented) before being sent.In other words, the value of MTU defines the maximum length of a data unit a protocol can send in one shot (without fragmenting)

Do we need to change MTU every time when the machine needs to communicate with other machines in different networks? Of course not, computer is very smart. With the help of PMTUD, we don’t need to change MTU manually. It can help detect the minimum MTU between different networks.

Path MTU Discovery (PMTUD)

It can help detect the minimum MTU between different networks. So no need to change MTU every time when the machine needs to communicate with other machines in different networks

For IPv4 packets, Path MTU Discovery works by setting the Don’t Fragment (DF) option bit in the IP headers of outgoing packets. Then, any device along the path whose MTU is smaller than the packet will drop it, and send back an Internet Control Message Protocol (ICMP) Fragmentation Needed (Type 3, Code 4) message containing its MTU, allowing the source host to reduce its Path MTU appropriately. The process is repeated until the MTU is small enough to traverse the entire path without fragmentation.

How to get MTU value?

 netsh int ip show int

For most Ethernet networks this is set to 1500 bytes

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We can see that the MTU is configured 1500 on my local are connection

How to find the real MTU in the network?

Open the command prompt

  1. Type the following: ping -f -l 1600 servername
  2. You should receive the message “Packet needs to be fragmented but DF set“, which indicates that the size of the packet sent (1600 bytes) was too large for the network to send without splitting it.
  3. Keep repeating step 2, but this time use smaller numeric values (such as 1500, 1450, 1400, 1350 etc.) for the size of the data packet.
  4. Eventually you should receive the ‘standard’ PING message “Reply from XX.XX.XX.XX: bytes=xxxx time=YYms TTL=ZZZ“. This means that this packet size was small enough to fit inside the MTU.
  5. Slowly increase the data packet size again, until you find the maximum size that can be sent without receiving the message “Packet needs to be fragmented but DF set“. This is your maximum MTU size between the client PC and the destination.

But usually we don’t need to do it. since the packet will be fragmented during transferring and the PMTU discovery will help detect the minimum MTU.

How to change MTU value in the windows ?

  1. Open the command prompt in elevated mode
  2. To see what interfaces you have  connected , type netsh int ip show int
  3. netsh interface ipv4 set subinterface “Local Area Connection” mtu=1500 store=persistent

where Local Area Connection is the name of the network connection on your computer, from the list obtained above and 1500 is the desired value for MTU

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Reboot after the changes done

More information

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How do I find my optimum MTU setting

http://www.dslreports.com/faq/5793

EnablePMTUDiscovery

http://technet.microsoft.com/en-us/library/cc957539.aspx

What is ‘MTU’ and how do I change it in Windows 2008 R2

http://craigocon.wordpress.com/2012/10/03/whats-mtu-and-how-do-i-change-it-in-windows-2008-r2/

 

Categories: Windows Servers

Server ‘c’ drive running out of space, what utilized most of disk space?

August 7, 2013 1 comment

Issue

Sometimes windows based server shows disk is full, but actual Disk space usage is not fully utilized

Disk space can be consumed by various factors. There are many files are hidden and protected by system. Protected files can’t be viewed even you are the administrator and its mostly designed for security purpose. If you can’t see the hidden or protected files with Read more…

Network Teaming in Windows Server 2012

November 29, 2012 1 comment

Network Teaming in Windows Server 2012

Windows server 2012 include native teaming of network adapters to provide load balancing, bandwidth aggregation and failover capabilities using standard Ethernet NICs.

Microsoft doesn’t provide native teaming on OS other than server 2012. In windows server 2012, it’s a built in feature

What are the benefits of having a NIC team?

Main benefits are Resiliency and Performance.

Resiliency

if any one of the network paths (or network cards) becomes disconnected or somehow fails, the remaining NICs in the team are in place to continue making sure that traffic is getting through.

Performance/bandwidth aggregation

It’s an interesting feature in my perspective. If server having 4 Ports* 1 GB , it can combine and get advantage of 4 GB bandwidth.

In normal scenario all standard server build with two NIC with 4 ports. We get only 1GB network bandwidth. By bandwidth aggregation we will combine all NIC in to one and get maximum bandwidth for NIC’s

How to setup teaming ?

Go to server Manager-Then Local Server

See the area I’ve marked in red? NIC Teaming by default is disabled. Select to Enabled ”. This brings up the below NIC Teaming window

Click On Tasks-And select New Team. That will bring up the New Team Window

I’ve named New Team, and selected two 10 GB of my NICs to become members of the team. Also, I’ve expanded the Additional Properties area to show you that you have additional options available for configuration.

Click OK, and the NIC teaming is configured ( see below)

In Network Connections window, where you’ll see all of the physical as well as team NICs.By teaming with 2*10 GB adapter I got 20 GB bandwidth on teamed adapter. It’s called bandwidth aggregation

For more info : http://technet.microsoft.com/en-us/library/hh831648.aspx

Categories: Windows Servers
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