Changing MTU size

N3tWrkNut

I have never changed the MTU size before. I am seeing a huge amount of Giant errors and think it needs to be changed to 9000. If you have changed this before does is pose any complications?

Thanks!

billscott92787

N3tWrkNut wrote: »

I have never changed the MTU size before. I am seeing a huge amount of Giant errors and think it needs to be changed to 9000. If you have changed this before does is pose any complications?

Thanks!

MTU is the maximum transmission unit. It defines the maximum size of A PDU that can be transmitted without performing fragmentation. I would say that this can cause issues in some scenarios. I am not sure of many examples from my experience. I do know that when I tried to change the MTU on my PS3 it informed me that it could cause some games not to function properly or they may not work online at all.


You are probably getting errors because it is creating Jumbo frames which are sizes bigger than 1500 which I believe is the Ethernet 2 industry standard.

mikem2te

N3tWrkNut wrote: »

I have never changed the MTU size before. I am seeing a huge amount of Giant errors and think it needs to be changed to 9000. If you have changed this before does is pose any complications?

Thanks!

Where are you thinking of changing it?

In my limited experience I have only ever changed MTU on a Windows PC, and that was to a lower value than the default of 1500.

We had an issue once when we were running IPSec VPN's over a broadband connection which would only support an MTU of 1500. After the IPSec headers were added some packets would be about 1540 bytes so we had unreliable data transmission. We had to lower the MTU's on the hosts (Windows NT) either side of the VPN to get any sort of reliability. More modern OS's allegedly have better auto MTU capabilites so this never occured using Windows XP or better.

Errors such as Giants and Runts are usually indicative of some network problem and changing the MTU may only mask the issue. Could be a full duplex / half duplex mismatch on a PC and switch or faulty NIC.

dtlokee

Which MTU are you looking to change? In terms of the L2 MTU it can be increased but only on gigabit interfaces, on 100mb/s it is limited to 1518 (or 1548 on most platforms to allow for additional L2 overhead like metro tags). You can't increase the L3 MTU to a value greater than the L2 MTU - L2 header value. This is usually 1500 for Ethernet.

GT-Rob

So you are worried there is an application trying to send 9000byte frames over your network? I would look into this application first as this should not be.

You can look into enabling jumbo frames on your switches, but I would do some investigating first.