help with jumbo frames

[madlemming]

We have a small network but do a lot of file transfers/backups, so I'm upgrading everything to gig-e and trying to get jumbo frames working.

We have a decent managed switch that supports jumbo frames, and a cheap router (for wifi/dhcp/gateway) that does not.

All pc's (which have jumbo turned on) are plugged into the switch (which also has jumbo turned on), and the switch is plugged into the router.

Naively, I expected to be able exchange jumbo packets between the jumbo-enabled devices on the switch, and anything going to wlan would get fragmented. This seems not to be the case... "ping -f -l 2000 " from one device on the switch to another fails, saying the packet must be fragmented.


First, should this configuration work at all? If so, can anyone suggest further troubleshooting?

[dwhess]

I have that kind of setup at home except with my wan traffic on a separate 10/100 network

Don't you have to set the IP stack MTU to 9000 or whatever your jumbo frame size is?

mturoute.exe works well for testing and drtcp.exe works well for adjusting the IP stack parameters.

Code: Select all

C:\>mturoute 192.168.168.1
* ICMP Fragmentation is not permitted. *
* Maximum payload is 10000 bytes. *
+ ICMP payload of 5046 bytes succeeded.
+ ICMP payload of 7523 bytes succeeded.
+ ICMP payload of 8761 bytes succeeded.
- ICMP payload of 9380 bytes failed..
- ICMP payload of 9070 bytes failed..
+ ICMP payload of 8915 bytes succeeded.
- ICMP payload of 8992 bytes failed..
+ ICMP payload of 8953 bytes succeeded.
+ ICMP payload of 8972 bytes succeeded.
- ICMP payload of 8982 bytes failed..
- ICMP payload of 8977 bytes failed..
- ICMP payload of 8974 bytes failed..
- ICMP payload of 8973 bytes failed..
+ ICMP payload of 8972 bytes succeeded.
+ ICMP payload of 8972 bytes succeeded.
Path MTU: 9000 bytes.

[crazybus]

You have to use netsh to change the MTU in Windows 7 [Howto].

[madlemming]

AhHa... of course I was doing testing for the one win7 machine we have. Working nicely now.

Thanks to you both, mturoute is very nice.