How Can I Get Two Separate VLANs On Layer 2 Switch To Communicate?
Let's take a step back and look at this from the switch's point of view. When a new device is connected to a layer 2 switch and sends out a frame, the switch receives that frame and dissects it. The switch then looks inside that frame for the source and destination devices MAC Address. When the switch finds the senders MAC Address the switch then compares it MAC Address Table to see if it has that device stored within its table
If the switch does not have that device it then adds it to its Mac Address Table with a corresponding port number.
The switch then looks at its Mac Address Table for the destination Mac address within its table.
If the switch finds the Mac address of the destination device it then forwards that frame out of the corresponding port
But what happens if the switch does not have the destination MAC address within its table? The switch then floods an Address Resolution Protocol or APR request out each port except the port that the frame comes in on and waits for a reply. For example the ARP may say "Who is MAC 0000.1111.2222.3333?" and that device would respond by saying "I am here". The switch would then note that MAC address and port number within its Mac Address table allowing it to quickly forward frame to devices on the same subnet.
Ok so what happens if i add another VLAN to the switch? If another vlan is added to the switch then a layer 3 device must be attached. This layer 3 device is normally a router or a switch capable of routing.
In the diagram below the switch is connected to a layer 3 device, in this case a router by a "trunk" link.
A trunk link carries separate VLAN traffic to a layer 3 capable device. To identify the VLAN that the frame belongs to the switch uses different techniques such as Inter link Switching (ISL) or 802.1Q
Now in order to allow different subnets to communicate a router or layer 3 device must be configured with the ip address of each gate way.
So how do you configure 1 interface with two ip address that carry separate vlan traffic?
This is done by using the sub interfaces.
Below is a screen print of how to configure two sub interfaces.
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