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New Member

MAC addresses and vlans

Can a cisco switch "see" multiple MAC address of off a single switch port. For example if I have two PC's connected to a small workgroup switch which uplinks to the cisco switch, will the cisco switch be able to determine that there are two machines and will it be able to tell which vlan each one belongs too?

10 REPLIES
New Member

Re: MAC addresses and vlans

I have my desktop and my laptop plugged into the same Catalyst switch port every day (sometimes via physical splitter, sometimes via 8-port workgroup switch). But I think that you configure physical ports into VLANs - not MAC addresses. So I think that both MACs must belong to the same VLAN if they belong to the same switch port.

New Member

Re: MAC addresses and vlans

I would agree...each switch port can be a member of only one vlan.

and yes the switch can see multiple mac addresses off of the same port, hence the reason they allow for so many addresses in the first place.

if you want to be able to dynamically change the vlan of the port depending on mac-address you could look into VMPS. i had limited success with it in the past, but i don't believe it is actually a cisco supported technology anymore...at least i don't believe they recommend anyone use it.

New Member

Re: MAC addresses and vlans

For your reference, findings based on my own personal experience. Cisco switches will see multiple mac addresses for all ports when executing the "sh cam dy" command. If there is a hub or a Cisco switch attached to one of its ports, the multiple mac addresses owned by the device attached will be displayed.

EXCEPTION.. CABLETRON SWITCH.. if a cabletron device is attached to the cisco switch, MAC addresses owned by the cabletron device will not be displayed on the cisco switch.

New Member

Re: MAC addresses and vlans

Yes.

New Member

Re: MAC addresses and vlans

Yes. You can see that with the 'show cam dyn x/x' on a Cat4000/5000/6000.

New Member

Re: MAC addresses and vlans

The cisco switch will see multiple VLANs but you will only be able to assign one VLAN / port. Thats how I have seen it work with a HUB anyway.

New Member

Re: MAC addresses and vlans

Suppose hosts A (port p1, Vlan v1) and B (port p2, Vlan v2) are connected to Switch S1.

First case: S1 is connected (port s1p3) to Switch S2 (port s2p3) thru a a trunk link (which carries Vlan v1 and v2). Now, for S2 to deliver a frame to A it has to forward it out of its s2p3. Since s2p3 is in trunking S2 has to specify A's Vlan (ISL encapsulation). Thus S2 -in this case- needs to know vlans of hosts attached to S1.

Second scenario: S1 is connected to S2 via 2 links one dedicated to each vlan: s1p4-s2p4, vlan v1 and s1p5-s2p5, vlan v2. In this case S2 can forward frame to A out of its s2p4 and to B out of its s2p5. Since those ports are not in trunking S2 does not need to know vlans.

So... S2 will know the table MACs--s2ports (even for MACs not directly connected to s2 ports). And if some MACs belonging to different vlans are reached thru trunk port s2 will also need to know the MAC--vlan table.

You may doublecheck this by reading the 'set cam' explanation under cat 5000 command reference.

Also thanks to Leo Avella

New Member

Re: MAC addresses and vlans

This will only work if the workgroup switch is vlan aware. That is it is able to support some sort of vlan trunking protocol like 802.1Q or ISL. The port must trunk all vlans attached to the workgroup switch and then make forwarding decisions at the workgroup switch.

New Member

Re: MAC addresses and vlans

Yes it can. You can use the "show cam dynamic" command to see all of the MAC Addresses the switch is aware of and what port it is associating them to. You can also use the same command to see what MAC Addresses are associated with a certain port or vlan. The port doesn't have to be a trunk. E.G. show cam dynamic mod/port or show cam dynamic

Chad

New Member

Re: MAC addresses and vlans

Yes, the switch can see multiple MAC address on a single port, but that port can only belong to one vlan at a time.

It's the port that belongs to the vlan not the pc.

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