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VLAN routing on a Layer3 Switch

tsmarcyes
Level 1
Level 1

I am a little confused on the details on how VLANs work at layer 3. Lets say you have a switch (regular,not layer 3) with 8 ports that is connected to an external router. Each port is configured with a different VLAN lets say. In order to route traffic between those VLANS, I would have to set up either ISL or 802.1.q tagging on the physical router interface connected to the switch. Correct me if I'm wrong, but then you have to set up subinterfaces on that physical router interface connected to the uplink port on the swtich for each VLAN(ie.subnet). So, since we have 8 ports in their own VLAN, that would be 8 subinterfaces, with each having their own ip addresses (subnet). Then you would have to set up on the switch (the uplink port), to have be a trunk port with all of your VLANS (1-8). You dont have to put any layer 3 information for the VLANS on the switch itself.

Now on a layer 3 switch, which I come to understand to be basically a router within the switch, how would i achieve the same result. After creating the VLANS on the switch ports, then what would I do. I am looking for some details..maybe not so much the actual commands, but tell me what I have to do for each port, whether I have to assign ip addresses to them, set up a tagging methode, etc...

1 Reply 1

Mark Turpin
Level 5
Level 5

Actually you're doing very well!!!

You've got the right mindset for how it works on a Layer2 Switch. So let me pick up at the Layer3 Switch part...

When you have a Layer 3 switch, your switch ports are configured with the Vlan they will belong to with the "switchport access vlan" command.

The Layer 3 interface is a virtual interface and is called - Vlan - which is configured like so:

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a008019e74e.shtml

You'll assign an IP address to the virtual interface (the Vlan interface) and the switch will then route internally between the vlans.

Let me know if this helps!

-Mark

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-Mark Turpin