#18590: MTU issue involving bridge or VLAN ?
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Reporter: ambrosa | Owner: developers
Type: defect | Status: new
Priority: high | Milestone: Chaos Calmer (trunk)
Component: base system | Version: Trunk
Resolution: | Keywords:
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Comment (by ambrosa):
My bridge setup:
{{{
root@openwrt:~# brctl show
bridge name bridge id STP enabled interfaces
br-lan 7fff.e8de2795e014 no eth0.1
wlan0
}}}
After reading this nice tutorial
http://www.microhowto.info/tutorials/802.1q.html
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''
Effect on the MTU
The MTU (maximum transmission unit) of a network interface is the size of
the largest block of data that can be sent as a single unit. The standard
Ethernet MTU is 1500 bytes at the network layer or 1518 bytes at the link
layer, the difference being due to the 14-byte header and 4-byte frame
check sequence that enclose the payload of an Ethernet frame.
On a VLAN trunk the need for each frame to be tagged adds a further 4
bytes of link-layer framing. This can be accommodated either by increasing
the link-layer MTU or by reducing the network-layer MTU:
To use the standard network-layer MTU of 1500 bytes, the equipment
must support a link-layer MTU of at least 1522 bytes.
If the link-layer MTU were limited to the standard value of 1518 bytes
then the network-layer MTU would need to be reduced to 1496 bytes to
compensate.
Devices with explicit VLAN support are supposed to accommodate a link-
layer MTU of at least 1522 bytes, but if you are using generic hardware
then it may be necessary to accept a lower value. All devices on a given
IP subnet must use the same network-layer MTU, so if you intend to deviate
from the standard value of 1500 bytes then you will need to configure all
affected machines
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After reading above it looks that for some strage reason link layer packet
size is not 'expanded' to 1522 bytes (as Barrier Braker does) and so we
need to 'reduce' frame length (MTU from 1500 to 1496) at client side.
This works but it's a workaroud.
Any solution at router side ?
--
Ticket URL: <https://dev.openwrt.org/ticket/18590#comment:1>
OpenWrt <http://openwrt.org>
Opensource Wireless Router Technology
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