mac addresses and vlans

carl_townshend · ‎05-30-2008

Hi all, when a switch learns a mac address, if I have vlans on the switch, does it constrain the mac address learnt to that vlan? is this how the isolation is done ?

royalblues · ‎05-30-2008

When a switch learns the mac-address on a port, it will also associate it to the vlan.

The switch thus know what all ports it needs to forward the traffic to when it receives a broadcast /unknown unicast for that vlan

Narayan

carl_townshend · ‎05-30-2008

so, do will a vlan stop my switch forwarding to mac addresses between vlans?, does the switch only allow mac addresses to communicate if they are on the same vlan, and will only forward between vlans at layer 3 ?

Wilson Samuel · ‎05-30-2008

Please find the answer to your query as below:

1.Will a vlan stop my switch forwarding to mac addresses between vlans?

Yes. A switch will not allow any communication between the VLANs unless there is an involvement of L-3 device.

2. the switch only allow mac addresses to communicate if they are on the same vlan, and will only forward between vlans at layer 3 ?

All the communications between two hosts (MAC Addresses at the L-2) will take place only if both of them are on same VLAN. However if the Switch is an L-3 Switch or if there is a router involved (using Trunk) then the Router or the L-3 engine will Route-Switch the packet.

HTH,

Please rate all helpful posts.

Kind Regards,

Thotsaphon Lueangwattanaphong · ‎05-30-2008

Carl,

You need a layer3 device to route between vlans. You can think that the vlan functions will separate the switch to be many logical switches. Switch will forward frames within the vlan. We use vlan to separate broadcast domain and to apply security policies.

HTH

Thot