Lectures on LAN Switching – Part 07

Connecting Different Switches

Let’s say you need to expand your current single-Switch network setup, and the current Switch you have is configured with multiple VLANs already. How can you accomplish this?

Well, in a single-VLAN Switch, you can easily do this by connecting a switchport from the original Switch to a switchport on the new Switch (since they belong to the same VLAN). The VLAN or the network segment will automatically extend to the other Switch without complex configuration.

But, what if you have multiple VLANs, just like the scenario given above? This scenario is more realistic given that most organizations will surely have different departments. These departments will represent the VLANs present on a Switch.

The easiest way for this scenario is for a switchport assigned to a specific VLAN, to be connected to a switchport on the other Switch that is assigned to the same VLAN. The same thing can be done to the others. So, if you have 10 VLANs configured, you’ll need to have 10 switchports (each assigned to a different VLAN) on one Switch, and connect them to 10 switchports on the other Switch.

Are you already seeing the problem? It’s quite obvious, right? =) By just extending the VLANs to the other switch, we already wasted a lot of switchports for the Switch-to-Switch connections. Can we just do it using a single link?

We’re in luck, because the answer is yes, and it can be done using Trunking.

Access vs. Trunk

Before we discuss Trunking any further, let’s discuss first the difference between the two most common switchport configurations for connecting Switches: Access mode and Trunk mode.

Access mode is the switchport mode used when you want to allow traffic only from a single VLAN. Packets coming from a Switch will pass through an Access link on its original form (no tagging involved). The receiving switcport (which is configured on the same VLAN) will assume that all packets coming in should be untagged and belongs to the VLAN configured to this switchport.

End devices like Personal Computers (PCs) also connect to a Switch thru Access ports. A PC’s Network Interface Card (NIC) should send and receive packets on its original form, given that PCs don’t normally understand any tagging concept, or even the concept of VLAN.

Trunk mode, on the other hand, is the switchport mode used when you want to allow traffic from different VLANs to pass through between Switches. Trunking uses the concept of tagging in order to identify a packet belonging to a certain VLAN. When a switchport sends a packet, it is automatically tagged with the VLAN ID before it goes out of the said switchport. On the other hand, the receiving switchport will remove and read the tagging or encapsulation, and assign it to the VLAN specified on the tag information.

With a trunking connection, the Switches gain the benefits of maximizing link utilization, as well as saving switchports from being wasted.