[Bridge] Re: Sorry for bothering Beg for help

Stephen Hemminger shemminger at osdl.org
Tue Nov 30 11:33:37 PST 2004

On Tue, 30 Nov 2004 21:50:00 -0800
"kingz" <jzhang at cienettechnologies.com> wrote:

> Hi,
> Sorry for bothering you!
> Now I am in deep trouble on Ethernet bridge and VLAN, I posted my issue,
> but no reply, so I only beg for your help!
> Could you give me any comment, hint, way to fix my issue? Thank you very
> much!
> The following is my issue:
> Now I am having trouble in Ethernet bridge and 802.1Q VLAN, I have the
> following network:
>    Internet
>        |
>        |
>   Gateway
>        |
>        |    <---------  If insert one Ethernet bridge box in here
> ,somethings changes
>        |
>      H u b    ( one normal hub or switch , no VLAN settings)
>       |   \
>       |     \
>   D-Link  802.1Q VLAN Switches (their uplink port-port 1  connected to
> the upper hub are tagged, those ports connected with PCs are untagged).
> Our client PCs connect to these VLAN Switches, they access Internet
> through that UPNP gateway, It works all OK; Every PC connects to one
> untagged port in a 802.1Q VLAN Switch , this untagged port and the
> tagged  uplink port (e.g. port 1) make up of one VLAN,  the tagged port
> 1 is a common uplink port , So from the gateway we can see these VLAN ID
> of PCs, so far it all work fine;  but when I insert one Ethernet bridge
> box ( one linux box, kernel 2.4.26, ebtables-brnf-6_vs_2.4.26 patch, one
> rt8139 NIC, one e100 NIC) between the gateway and the Hub, somethings
> changed: 
> I was not able to log  in hotmail, maybe also on certain websites. 
> For example, I access  www.hotmail.com <http://www.hotmail.com/> , it
> loads OK. When I input my account, and hit Login, it goes to a error
> page(DNS error or server not found)  after a moment. Except the above
> site, maybe also have others, but most sites work fine, very strange! I
> am sure that this Linux Ethernet bridge is only transparent bridge, no
> any Iptables filter and rule. 
> If I remove the tag for VLAN(just change the port 1 to untagged),  also
> with this Linux Ethernet bridge it works all OK, also have no hotmail
> login error. Very strange, it as if the issue is on VLAN , tag, or Linux
> Ethernet  bridge , I have no idea of it now! 
> Thanks!
>  BR 
> Kingz

What is happening is that the tagged frames are longer (1504) bytes and so they
get dropped.  If you want all frames to go through the bridge, then you need
to increase the MTU of both interfaces to 1504. Also, both drivers have to be able
to accept bigger (tagged) packets.

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