An Engineers Notebook

Enabling Layer 3 switching on a Cisco switches

Step 1.

If you have a layer 3 Cisco switch and want to route packets via different VLANs, you need to enable this. 

First do a “Show run” and check to see if:

ip routing

is present. If it is then you’re good to go with step 2. If not, just run the command “ip routing“.

Step 2.

After creating you VLANs the normal layer 2 way like this:

vlan 10

name NetworkA

You next need to create a VLAN interface like this:

interface VLAN 10

ip adderss


Finally, you need to add the ports that need to route together to the interface.

If you’re connecting a DHCP server to the switch to provide DHCP services, this link should be a trunk port with access to the correct VLANs to provide DHCP. You’d then need to assign an ip helper on the trunk port to point to the DHCP server. This tells the switch to forward any DHCP broadcasts to the DHCP server for any traffic reaching the port. Since the port is a trunk all VLAN traffic is tagged and gets sent to the DHCP server.


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