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Re: [lwip-users] Add support for outgoing VLAN tags?

From: web
Subject: Re: [lwip-users] Add support for outgoing VLAN tags?
Date: Wed, 09 Nov 2011 18:13:46 +0100

On 9 nov 2011 14:37 "Dance, Brian" <address@hidden> wrote:
On a Windows machine I have a VLAN capable Network Card. When the (Intel) driver is configured for VLAN use the windows control panel shows a network interface for each vlan. Programs using sockets are unaffected.

In the lwip contect the netif structure at the equivalent level to the Windows netowrk interface. I think this means that there is a possible vlan output implementation for lwip based on adding vlan/qos info to netif without any socket changes.

Below the socket level it will be necessary to locate the correct netif to use for output. I would expect different vlans to be using different subnets so ip routing can determine which netif to use using destination ip address.

What you are talking about is using VLAN tags to divide your netowrk into several different virtual networks, which is perhaps the most common use of VLAN tags.
But what not everyone knows is that the VLAN tags can also be used to enable QoS at the Ethernet level, WITHOUT splitting up the network into different virtual networks.
This is done by using the VLAN ID zero.
I don't think windows machines supports this feature. And perhaps there wouldn't be much point since windows has pretty horrible real time properties anyway.
On Linux it looks like it may be supported, but I have not tried it in practice.

For the industrial protocol "EtherNet/IP", Ethernet QoS is mandatory at least for all devices which supports certain features (such as DLR redundancy and/or CIP sync).
Very simple EtherNet/IP devices may perhaps get by without it, but I would have to look it up in the EtherNet/IP specification to say for sure.

Timmy Brolin

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