mtu size

[YLTan]

Does anyone know the advantage of setting mtu size to 9000 as compare to 1500 in HP highend server such as SuperDome?

When do I need to use mtu size 9000 and when is it better to use 1500?

Is mtu size 4500 allowable?

[RAC_1]

mtu size of 4500 or 9000 allowable. but your network needs to support it.

The default for hp is 1500.

you can add it for specific route with route command. -p option is the mtu option.

[Ravi_8 · Accepted solution]

Hi,

Using MTU values is depend of the network you have.
The MTU can be either the standard Ethernet MTU (1500 bytes) as specified by the IEEE 802.3z or the Jumbo Frame MTU (9000 bytes). Because the total length of the frame increases, the single frame overhead stays constant, however there is a reduction in the number of frames required to transfer a given amount of data. This results in a more efficient frame.

For 100BT adapters, i.e., those adapters controlled by the btlan driver, the MTU can be set as low as 257 bytes. Any lower than that is not supported.

For Gigabit Ethernet adapters, i.e., those controlled by gelan or igelan, the MTU can only be set to 1500 or 9000. Other values are not supported.

You can determine the adapter type by using ioscan

Also, note that lanadmin does not check for the minimum MTU size. The driver checks for it. So, filing a defect against lanadmin may not be effective.

[Steven E. Protter]

The setting 1500 is standard. Messing with it will not improve performance, though your 4500 setting is easily alloweable.

Before playing with this settng, you might want an actual story of how it was used.

We were having networking problems getting traffic in and out of one of my HP-UX boxes from a remote site, through a router.

We changed the mtu size for some long term ping testing, trying to figure out if there really was a networking problem.

The largere packets better simulated the kind of data we were supposed to be lobbing back and forth over the WAN.

As it turned out the ping times were not consistent due to a problem on the remote router. The larger packets made this easier to diagnose.

SEP

[rick jones]

I wasn't aware that HP had started supporting a 4500 byte MTU.

Regardless, _only_ the Gigiabit interfaces support the "jumbo frame" MTU's, and then, only when operating in gigabit mode, not 100BT.

If you set the MTU to 9000 on one system, _all_ the other systems in the same broadcast domain (that is all the machines joind simply by switches, rather than routers), and the switches, must be set to the same 9000 byte MTU. Otherwise, you will start to have connectivity problems. Particularly with UDP comms (NFS comes to mind).

Using a larger MTU can indeed greatly improve the performance for bulk throughput - by reducing the number of packets required to transfer a given quantity of data. Reduce the number of packets, reduce the CPU overhead...

It won't do anything for apps which never send more than 1500 (well, 1460) bytes at a time.