OVERKILL
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Since this topic is fresh in my mind, as I just did it on my 1841, I figured I might as well make it a topic
MTU; Maximum Transmission Unit
MSS; Maximum Segment Size. This is basically your MTU -40 bytes of the TCP and IP headers.
Most users will only have to worry about the top one.
The most affected by this are users of PPPoE. The encapsulation uses 8 bytes of the total packet size. But depending on how your ISP has your equipment configured, setting your MTU lower than this may be a step needed to prevent unnecessary fragmentation.
Excessive fragmentation can lead to web pages not fully loading, "laggy" browsing and some sites not loading at all.
For example, the MTU for my ISP is 1452. If I leave it at 1492, I can't bring up Microsoft's website. But many others DO work. Setting my mss to 1412 and MTU to 1452
Quote:
config terminal
interface Dialer0
ip mtu 1452
ip tcp adjust-mss 1412
significantly reduces the latency of my browsing and gets rid of the load errors.
Determining your maximum MTU is easy:
In Windows XP: Start -> Run -> CMD
In Windows Vista/7: Start ball -> Find box -> CMD
in the command prompt that opens: ping www.thewebsiteforyourisphere.com
This is to verify your isp responds to ping requests. If not, find a site that does for the next step.
ping -f -l 1472 www.yoursitethatpings.com
The meaning of the flags are as follows:
-f don't fragment the packet
-l the value following this flag is the size of the packet you wish to send.
We are using 1472 because this is 1500, which is the maximum MTU size, removing 20 bytes for the IP header and 8 bytes for the ICMP header (ping).
Many will be greeted with this message in the terminal:
Quote:
Packet needs to be fragmented but DF set.
Hit the "up" arrow on your keyboard to bring back your ping command and reduce the size of the packet by 2. Hit enter.
If you receive the same message, repeat.
Do this until you do not get the error message.
Take that value, add 28 to it and you have your MTU value for your ISP.
Login to your router, find the MTU (on Linksys routers, this is right on the first page for example) and change it to this number.
Save your settings.
Enjoy.
MTU; Maximum Transmission Unit
MSS; Maximum Segment Size. This is basically your MTU -40 bytes of the TCP and IP headers.
Most users will only have to worry about the top one.
The most affected by this are users of PPPoE. The encapsulation uses 8 bytes of the total packet size. But depending on how your ISP has your equipment configured, setting your MTU lower than this may be a step needed to prevent unnecessary fragmentation.
Excessive fragmentation can lead to web pages not fully loading, "laggy" browsing and some sites not loading at all.
For example, the MTU for my ISP is 1452. If I leave it at 1492, I can't bring up Microsoft's website. But many others DO work. Setting my mss to 1412 and MTU to 1452
Quote:
config terminal
interface Dialer0
ip mtu 1452
ip tcp adjust-mss 1412
significantly reduces the latency of my browsing and gets rid of the load errors.
Determining your maximum MTU is easy:
In Windows XP: Start -> Run -> CMD
In Windows Vista/7: Start ball -> Find box -> CMD
in the command prompt that opens: ping www.thewebsiteforyourisphere.com
This is to verify your isp responds to ping requests. If not, find a site that does for the next step.
ping -f -l 1472 www.yoursitethatpings.com
The meaning of the flags are as follows:
-f don't fragment the packet
-l the value following this flag is the size of the packet you wish to send.
We are using 1472 because this is 1500, which is the maximum MTU size, removing 20 bytes for the IP header and 8 bytes for the ICMP header (ping).
Many will be greeted with this message in the terminal:
Quote:
Packet needs to be fragmented but DF set.
Hit the "up" arrow on your keyboard to bring back your ping command and reduce the size of the packet by 2. Hit enter.
If you receive the same message, repeat.
Do this until you do not get the error message.
Take that value, add 28 to it and you have your MTU value for your ISP.
Login to your router, find the MTU (on Linksys routers, this is right on the first page for example) and change it to this number.
Save your settings.
Enjoy.
