Cannot set MTU size larger than 2K

[JFMG]

Hi everyone: I have the following setup:
- Host OS: Windows 7.
- Guest OS: CentOS Linux 7.
- Bridged adapter:

• - Host NIC: Intel(R) 82579LM Gigabit Network Connection.
  - Adapter type: either "Intel PRO/1000 T Server (82543GC)" or "Paravirtualized Network (virtio-net)" (I get the same results with both, worse results with the rest).

I'm trying to set the MTU size to 9000. Using the rest of adapter types, I cannot change it from 1500. With those two adapter types I'm able to do the change ("ip a" command shows "mtu 9000") but I'm only able to do ping in both directions (from the host to the guest and vice versa) with up to 2005 bytes (actual ICMP packet size: 2047).

I don't know where this 2K limitation comes from. Any hint from the experts will be more than welcome.

Thanks.

[socratis]

I'm not an expert in networking by any means, but I would expect the ping to have a small payload, no?

[JFMG]

Hi socratis: yes, ping uses small packets but I'm using a larger ping packet size (-s option in Linux, -l option in Windows) just to test large packets.

Without changing the MTU in the interface, I can use packets of up to 1472 in ping. When I change the MTU size (ip link set eth1 mtu 9000), I should be able to use in ping packets close to 9000 bytes but I can get through with packets up to 2005 bytes only.

By the way, I've tried using Fedora 10 as guest OS and I get the same results, i.e., limitation under 2 KB.

[JFMG]

Hi again: isn't there anyone using MTUs larger than 2 KB with a Linux guest and a Windows host?

[JFMG]

Hi: this behavior was reported as a bug (https://www.virtualbox.org/ticket/15140) and fixed in v5.0.15-105511 test build.