mac addresses switching between vlans

Unanswered Question
Jun 3rd, 2008

Hi all, Am i right in saying at layer 2, 2 mac addresses can only see each other in the same vlan, if one was in another vlan, layer 3 would need to be involved ?

I have this problem too.
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Ryan Carretta Tue, 06/03/2008 - 01:12

The simple answer is yes, vlans represent broadcast domains so you would need to route a packet to get from one vlan to another. Routing would require a layer-3 header.

That said, through some questionable methods, you can circumvent some of these necessities (proxy-arp, vlan bridging, etc.)

grichardson661 Tue, 06/03/2008 - 01:20

Correct. Machines in the same VLAN can communicate with each other with out layer 3 implementation.

To communicate with hosts in different VLANS you will need either, a router with multiple Ethernet interfaces, or a router that supports Trunking or a layer 3 Switch.

Jon Marshall Tue, 06/03/2008 - 01:30


It is a little bit more complicated than that depending on what you mean by Layer 3. If by layer 3 you mean routing then yes you are correct.

But IP addresses "live" at layer 3 and IP addresses are always involved when we talk about mac-addresses/IP addresses. What i mean is

Host1 =

Host2 =

They are both on the same subnet. Host1 wants to send a packet to Host2. It needs it's mac-address to do this. But first Host1 needs to work out if Host2 is on the same subnet and it does this by using IP addresses ie.

Host1 -> network =

So Host1 knows it's network is When it wants to send a packet to Host2 it compares Host2's IP address with it's own subnet mask ie.

Host2 -> network =

So Host1 knows that Host2 is on the same network. Then it arps out for Host2 mac-address ie. Host1 broadcasts out - "Who has the mac-address for".

So layer 3 is always involved from an IP perspective.


carl_townshend Tue, 06/03/2008 - 01:36

basically, the situation is this, we have a server that talks to devices via mac address only, no ip involved, but we want to put the server in another vlan and change the ip address, but it will then lose the comms to these devices as the mac will no longer be visable in that vlan, is there anything I can do here ?

Ryan Carretta Tue, 06/03/2008 - 01:41

There isn't much to be done if that truly is the case...but I almost find that kind of hard to believe. What kind of application are you using that operates exclusively at layer-2?

carl_townshend Tue, 06/03/2008 - 02:36

as mentioned in another post, they use T_SAP addressing, from the config I have seen I can only see mac addresses, the systems are many years old

paul.matthews Tue, 06/03/2008 - 03:04

I assume this is linked to your other post?

What systems are they? That may help us make a suggestion. An indication of just how old would help, as that may give a clue on available options.


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