mac addresses switching between vlans

carl_townshend · ‎06-03-2008

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 ?

Ryan Carretta · ‎06-03-2008

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 · ‎06-03-2008

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 · ‎06-03-2008

Carl

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 = 192.168.5.10/24

Host2 = 192.168.5.11/24

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 192.168.5.10 255.255.255.0 -> network = 192.168.5.0

So Host1 knows it's network is 192.168.5.0. When it wants to send a packet to Host2 it compares Host2's IP address with it's own subnet mask ie.

Host2 192.168.5.11 255.255.255.0 -> network = 192.168.5.0

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 192.168.5.11".

So layer 3 is always involved from an IP perspective.

Jon

carl_townshend · ‎06-03-2008

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 · ‎06-03-2008

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 · ‎06-03-2008

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 · ‎06-03-2008

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.