Help! My whole network seems capped at 1300 MTU
Posted: Wed Feb 16, 2011 9:10 am
I am at a loss here and help would be greatly appreciated.
To start, here is my network configuration:
Cable Modem <-> Linksys WRT54G <-> Netgear GS105 (gigabit switch) <-> wired devices (disregard wireless devices for now)
Two of the devices (a Win7 desktop and a NAS) are gigabit; the others (a printer and a laptop) are fast ethernet. I have the gigabit devices connected through the gigabit switch so that the desktop and the NAS can talk to each other using gigabit (which they do; transfers between them are faster than 100Mbps). The printer is connected directly to the router.
Here's my issue. The gigabit switch is supposed to support jumbo frames up to 9K, so I set both the desktop's and the NAS's MTU to 9K. To test it, from the desktop I pinged the NAS using "ping -f -l 9000 <ip>" and got the fragmentation message. I then tried pinging with 1500: same message. I have to go all the way down to 1272 (i.e., 1300 counting the TCP overhead) to ping without fragmentation! I also tried pinging the laptop from the desktop and got a max of 1272 as well. And vice versa. And pinging the NAS from the laptop. All combinations lead to 1272.
Thinking it might be a problem with the gigabit switch, I connected the desktop and laptop directly to the router--same result. The router's MTU setting is the default: 1500 (which shouldn't have mattered anyway when the devices were on the gigabit switch, bypassing the router).
Finally, I tried DSLreports' tweak tool (which looks at your WAN connection) and it indicated that I was maxed out at 1300. In fact it suggested that I "raise my MTU to 1500," which means that, from their end, it seems that I'm manually dialed back to 1300. But again, even if the cable modem or router was set to 1300, it wouldn't have affected LAN-only transfers through the gigabit switch. (Or would it? The router is the DHCP server--could it be throttling the computers when it assigns thier IP addresses?)
I started off wanting 9K jumbo frames but now I'll settle for 1.5K normal frames! Does anyone have any insights?
To start, here is my network configuration:
Cable Modem <-> Linksys WRT54G <-> Netgear GS105 (gigabit switch) <-> wired devices (disregard wireless devices for now)
Two of the devices (a Win7 desktop and a NAS) are gigabit; the others (a printer and a laptop) are fast ethernet. I have the gigabit devices connected through the gigabit switch so that the desktop and the NAS can talk to each other using gigabit (which they do; transfers between them are faster than 100Mbps). The printer is connected directly to the router.
Here's my issue. The gigabit switch is supposed to support jumbo frames up to 9K, so I set both the desktop's and the NAS's MTU to 9K. To test it, from the desktop I pinged the NAS using "ping -f -l 9000 <ip>" and got the fragmentation message. I then tried pinging with 1500: same message. I have to go all the way down to 1272 (i.e., 1300 counting the TCP overhead) to ping without fragmentation! I also tried pinging the laptop from the desktop and got a max of 1272 as well. And vice versa. And pinging the NAS from the laptop. All combinations lead to 1272.
Thinking it might be a problem with the gigabit switch, I connected the desktop and laptop directly to the router--same result. The router's MTU setting is the default: 1500 (which shouldn't have mattered anyway when the devices were on the gigabit switch, bypassing the router).
Finally, I tried DSLreports' tweak tool (which looks at your WAN connection) and it indicated that I was maxed out at 1300. In fact it suggested that I "raise my MTU to 1500," which means that, from their end, it seems that I'm manually dialed back to 1300. But again, even if the cable modem or router was set to 1300, it wouldn't have affected LAN-only transfers through the gigabit switch. (Or would it? The router is the DHCP server--could it be throttling the computers when it assigns thier IP addresses?)
I started off wanting 9K jumbo frames but now I'll settle for 1.5K normal frames! Does anyone have any insights?