Hi,
Some time ago i saw this patch:
http://permalink.gmane.org/gmane.comp.embedded.openwrt.devel/14678
and tried to increase the MTU of a D-Link Dir-615 rev E4 router. It
worked up to 1518, the value defined in the patch, but with higher
values i could set the mtu with ifconfig but the router couldn't send
frames with MTU higher than 1518. The Dir-615 rev E4 has a AR7240 SoC.
I also tested the reception of big frames with the default 1500 MTU on
the router. From a PC with a high MTU, i sent pings of increasing size.
With values higher than 1472 ICMP payload (or 1500 MTU) the router
replied the ping with fragmented packets. However with values higher
than 1490 ICMP payload (or 1518 MTU) the router stopped replying the
pings. I think this test shows the physical limitation of the SoC,
regardless of the MTU configuration, right? I did this same test with a
TP-Link TL-WR741ND, with the same result.
Best regards
Gabriel
El 15/09/2012 06:11 a.m., Antonio Quartulli escribió:
On Fri, Sep 14, 2012 at 01:58:27 +0200, Felix Fietkau wrote:
On 2012-09-14 9:14 AM, Antonio Quartulli wrote:
This patch introduces the possibility to modify the MTU of the Ethernet
interface with a maximum value of 2024 (as reported in RouterOS). The real MTU
value applicable to the interface is computed as difference between the maximum
frame length (2024) minus some overhead.
The default MTU size is unchanged; if large frames are desired, ifconfig
must be used to set the new MTU.
I think we need to do some thorough tests with different chips. From
experiments I remember that the behavior when sending and receiving
large frames is different in ar716x vs ar724x. Some chips can handle
bigger frames than others, some don't respond well to anything that's
30-40 bytes longer than the regular packet size.
Here[1] we have the values of the supported MTU per board. For the board that we
know what chip is used we could customize the driver and set the proper MTU?
E.g. I have a RB2011L and this board uses the AG71xx driver for eth0 and eth1,
but they have different MTUs. How can I find out what chipset each interface
uses? Once I know this I could parametrize the driver.
Cheer,
[1]
http://wiki.mikrotik.com/wiki/Manual:Maximum_Transmission_Unit_on_RouterBoards
- Felix
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