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New Member

Switch &VLAN

Hello everyone

I'm a freshman in this forum and also in the cisco career certification. I just passed the CCNA exam last week, but there are still some quetions confusing me, Would you like to give me some help? Thanks in advance

First is about switch: Does each port in a switch have it's own MAC address?

if does, will the receiving port change the source address in the frame through it to another port? if doesn't , then in a network with two or more switches, how to deal with the data transmitting through many switches?

Second is about VLAN: what is the main difference between the VLAN and the subnet in conception , in functionality, and in practice?


Re: Switch &VLAN

Switches have one mac address commonly. Unless the data is destined for the switch itself it changes nothing in the frame. Just builds switching tables (MAC/port).

VLAN: Layer 2

Subnet: Layer 3

Normally a VLAN can be seen as a grouping of L2 ports/switches that encompass one broadcast domain and a subnet is a L3 concept which lives in this broadcast domain. Switches provide L2 connectivity while routers allow communication between L3 subnetworks.

Hope this helps,


New Member

Re: Switch &VLAN

Thanks Don

But can you give some more details about the switching process in a switch ?

Thanks again

Re: Switch &VLAN


each port in a switch have it's own MAC address.

If you try sh int on Cat3548, you'll see:

VLAN1 is up, line protocol is up

Hardware is CPU Interface, address is 0004.c15c.5d00 (bia 0004.c15c.5d00)



FastEthernet0/1 is down, line protocol is down

Hardware is Fast Ethernet, address is 0004.c15c.5d01 (bia 0004.c15c.5d01)



FastEthernet0/48 is down, line protocol is down

Hardware is Fast Ethernet, address is 0004.c15c.5d30 (bia 0004.c15c.5d30)

But these addresses are used only for the communication starting or ending on the switch itself. They are not used when a frame is passing through the switch.

Another example:

if you try sh spantree on a CatOS switch you'll see there are different MAC addresses used for BPDUs sent in different VLANs.




Re: Switch &VLAN


each switch port has the mac address. but it is used in the switches communication like spanning-tree etc. When the switches received the packets, like looks the destination mac address, if the switches know the mac address it forwards to that port which it is pointing to,if not, it will forward to all the ports except the one he learned itself. and if the source mac address is not present in the bridge table, it 'll put the entry for the source mac address.

Vlan separeates the broadcast domain- vlan is just like the interface on the router- used to be each switch can part of one lan- broadcast domain but with vlans, one switch can part of many vlans.. you can define virtual vlans (VLAN) upto 1024 per switch like cat6000 - which is not possible one router.

Here is some url for your ref..

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