Assign multiple VLAN id to the same port

kinglizard86 · 12-05-2012, 11:50 AM

Hi all,

is it possible to assign multiple VIDs to the same port using brctl and vconfig?

Suppose you have a linux box with three ethernet interfaces (eth0, eth1, eth2). I want to configure eth0 in trunking mode with VID 10,20,30 and put eth1 on VLAN 20 and eth2 on VLAN 30. This is achievable with the following commands:

Code:

vconfig add eth0 20
vconfig add eth0 30
brctl addbr br20
brctl addbr br30
brctl addif br20 eth0.20
brctl addif br20 eth1
brctl addif br30 eth0.30
brctl addif br30 eth2

This way packets tagged with 10 that arrives at eth0 are sent out untagged on eth1 and packets tagged with 20 that arrives to eth0 are sent out untagged on eth2.

Now if I want to assign both eth1 and eth2 also to VLAN 10 what should I do? And is it possible or not?

Thanks.

nini09 · 12-06-2012, 12:13 AM

Yes, an interface can belong to multi VLAN and a VLAN can include multi interface.

kinglizard86 · 12-06-2012, 03:23 AM

Ok. But, how can I practically achieve that with vconfig and brctl? I have different solutions in my mind, but all of them imply that one interface should belong to two bridges, and I know that this is not possible.
For example, I could create a third VLAN virtual interface on eth0:

Code:

 vconfig add eth0 10

and a new bridge to handle that VLAN

Code:

brctl addbr br10 
brctl addif br10 eth0.10

But I can't add eth1 or eth2 to br10 because they already belong to another bridge.

nini09 · 12-06-2012, 02:10 PM

An interface can only be added into one bridge. Why do you want to add one interface into two bridge?

kinglizard86 · 12-06-2012, 02:39 PM

I think that the way to handle VLANs in a linux box with multiple interfaces is to create one bridge for each VLAN you want to manage (e.g. br10, br20..)and then add to each bridge the interfaces you want to be on that VLAN. Maybe my hypothesis is wrong.
But I have another question. Suppose to have one interface belonging to two different VLANs. How untagged packets that enter to that interface are handled? Toward which VLAN are they forwarded?

nini09 · 12-07-2012, 02:51 PM

Linux will support tagged and untagged interfaces. The eth0 interface is untagged. All vconfiged interfaces (eth0.n) are tagged. The untagged packets are handled by real physical interface.

eantoranz · 12-07-2012, 03:14 PM

This is some very nice crap that I haven't studied much in linux. That means you can have both tagged and untagged traffic going out of a single physical interface/link? Cool!

eantoranz · 12-07-2012, 03:16 PM

how about when you want to have one IP in certain vlan to which you have two interfaces connected? should bridge the vconfiged interfaces and set the IP address on the bridge, right?

nini09 · 12-09-2012, 04:50 PM

Yes, you can have both tagged and untagged traffic on same physical interface.

kinglizard86 · 12-13-2012, 03:47 AM

Quote:

Originally Posted by nini09

Yes, an interface can belong to multi VLAN and a VLAN can include multi interface.

In order to belong to multiple VLAN an interface must be tagged. Otherwise, how does it know to what VLAN incoming untagged packets belong to?

nini09 · 12-13-2012, 02:15 PM

That's why I say, the untagged packets are handled by real physical interface.