What are the advantages and disadvantages of using subinterfaces for inter-VLAN routing?

2 How do subinterfaces work?

Subinterfaces work by using 802.1Q encapsulation, which is a standard protocol for tagging frames with VLAN IDs. When a switch receives a frame from a device on a VLAN, it adds a 4-byte header to the frame that contains the VLAN ID and a priority value. This header is called the 802.1Q tag. The switch then sends the tagged frame to the router or the layer 3 switch that has the subinterface configured for that VLAN. The router or the layer 3 switch examines the 802.1Q tag and determines which subinterface to forward the frame to. The router or the layer 3 switch then strips off the 802.1Q tag and routes the frame to the destination IP address. If the destination IP address belongs to a different VLAN, the router or the layer 3 switch adds a new 802.1Q tag to the frame and sends it back to the switch that has the subinterface configured for that VLAN.

3 What are the advantages of using subinterfaces?

Using subinterfaces for inter-VLAN routing offers several advantages, such as saving physical ports and hardware costs, simplifying configuration and network design, and enhancing security. Subinterfaces enable you to connect multiple VLANs to a single physical interface on a router or layer 3 switch, configure inter-VLAN routing on a single device, and control which VLANs can communicate with each other. Additionally, access control lists (ACLs) or firewall rules can be applied to each subinterface for further security.

4 What are the disadvantages of using subinterfaces?

Using subinterfaces for inter-VLAN routing has some drawbacks, such as increasing CPU load on the router or the layer 3 switch that performs inter-VLAN routing, introducing a single point of failure, and requiring trunking and tagging. This can affect the performance and scalability of your network, especially if you have a lot of VLANs and traffic. Moreover, if the router or the layer 3 switch that has the subinterfaces fails, all the VLANs connected to it will lose connectivity, resulting in network downtime and disruption. Additionally, this can limit your choice of switch models and vendors and increase the complexity and overhead of your network configuration.