Q: How to change to MTU 9000 Jumbo Frames ?

fortechitsolutions

Well-Known Member
Jun 4, 2008
384
35
48
Hi all, sorry if this is 'blazing obvious' but I'm still getting the feel of this.

I have 2 Proxmox VE machines connected via gig-ether with intel e1000 NICs. All gear (2 servers, the switch) support jumbo frame MTU=9000

I can issue as root on console of the ProxmoxVE servers a command thus,

ifconfig eth0 mtu 9000

and this works fine. The single (Default config) bridged interface is bumped up to MTU 9000 as well.

I then try to make this a persistent configuration change, by editing the file,

/etc/network/interfaces

and adding "mtu 9000" to the stanza for the bridge interface, so things appear thus:

pvm1:/etc/network# more interfaces
auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
address 10.10.2.11
netmask 255.255.255.0
gateway 10.10.2.1
mtu 9000
bridge_ports eth0
bridge_stp off
bridge_fd 0
pvm1:/etc/network#


However when I restart network services, an error is returned and the MTU isn't set. I assume this is because I can't set MTU on the vmbr0 device (it is a bridge against eth0) - rather, the setting needs to be made on eth0 itself.

Alas, I can't really see here how eth0 is being configured at all (?!) - it isn't cited in the 'interfaces' file....

Is this a matter, that I need to add a section to the interfaces file for eth0 - where I specify only the MTU parameter.. and then let this be inherited by the vmbr device... (?)

Any pointers / suggestions are most welcome.

Thanks,


---Tim
 

diaolin

Active Member
Jul 7, 2008
300
3
38
Trentino Italy
mtu 9000

and adding "mtu 9000" to the stanza for the bridge interface, so things appear thus:

pvm1:/etc/network# more interfaces
auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
address 10.10.2.11
netmask 255.255.255.0
gateway 10.10.2.1
mtu 9000
bridge_ports eth0
bridge_stp off
bridge_fd 0
pvm1:/etc/network#


However when I restart network services, an error is returned and the MTU isn't set. I assume this is because I can't set MTU on the vmbr0 device (it is a bridge against eth0) - rather, the setting needs to be made on eth0 itself.


---Tim

Do so:

pvm1:/etc/network

:interfaces

auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
address 10.10.2.11
netmask 255.255.255.0
gateway 10.10.2.1
#mtu 9000
bridge_ports eth0
bridge_stp off
bridge_fd 0
up "ifconfig eth0 mtu 9000||true"

pvm1:/etc/network#

Ciao, Diaolin
 

fortechitsolutions

Well-Known Member
Jun 4, 2008
384
35
48
Hi,

Many thanks for the reply. Alas, I'm not certain why but this doesn't appear to work. It returns an error, 'command not found', when I make this change and restart networking.

What I see is,

--paste--
pvm1:/etc/network# vi interfaces

pvm1:/etc/network# /etc/init.d/networking restart
Reconfiguring network interfaces...device vmbr0 already exists; can't create bridge with the same name

Waiting for vmbr0 to get ready (MAXWAIT is 2 seconds).

/bin/sh: ifconfig eth0 mtu 9000||true: command not found

Failed to bring up vmbr0.
done.


What I've got in interfaces file is now, as per your suggestion:

pvm1:/etc/network# more interfaces
auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
address 10.10.2.11
netmask 255.255.255.0
gateway 10.10.2.1
bridge_ports eth0
bridge_stp off
bridge_fd 0
up "ifconfig eth0 mtu 9000||true"


--endpaste--


If you have any further comments or thoughts; certainly they are appreciated.


Thanks for your help with this ..

--Tim
 

fortechitsolutions

Well-Known Member
Jun 4, 2008
384
35
48
Hi,

Bit of an update. I did a bit more digging with google, and after a few trial and error kicks at this, I think it now works as desired. Thanks for your suggestion - it is what got me there :)

--Tim


---paste---
CURRENT WORKING CONFIG:

pvm1:/etc/network# more interfaces
auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
address 10.10.2.11
netmask 255.255.255.0
gateway 10.10.2.1
bridge_ports eth0
bridge_stp off
bridge_fd 0
pre-up ifconfig eth0 mtu 9000
#
# Note these next 2 lines are failed attempts that did not work..
# pre-up "/sbin/ifconfig eth0 mtu 9000"
#up "ifconfig eth0 mtu 9000||true"

Now we see:
pvm1:/etc/network# /etc/init.d/networking restart
Reconfiguring network interfaces...
Waiting for vmbr0 to get ready (MAXWAIT is 2 seconds).
done.
pvm1:/etc/network#

and the inteface has MTU 9000 shown:


pvm1:/etc/network# ifconfig -a
eth0 Link encap:Ethernet HWaddr 00:15:17:8E:45:3D
inet6 addr: fe80::215:17ff:fe8e:453d/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1
RX packets:335414 errors:0 dropped:12 overruns:0 frame:0
TX packets:376859 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:108382139 (103.3 MiB) TX bytes:242330024 (231.1 MiB)
Memory:e3b00000-e3b20000
...etc...
 

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