Routing between VLANs with subinterfaces

Routing between VLANs with subinterfaces

Because VLANs are functionally equal to multiple separate switches each with its own subnet, a router is required to route traffic between them. Networks that have multiple switches connected to a router of course need one physical connection from each switch to a separate port on the router. The same would normally be true of VLANs within a single switch, you would need a separate physical connection from a port in each VLAN on the switch, to multiple ports on the router. This becomes a problem for a number of reasons, but primarily because most routers don't have more than a few Ethernet ports, and even with enough ports this would be an unnecessary waste of resources on the router. The solution to this multiple connection problem is to use what is called a trunk line.

A trunk line aggregates traffic from multiple independent VLANs into a single physical connection between switches, or between a switch and a router. Within this trunk there is a logical division of the connection, by encapsulating each frame with VLAN information, or by using a special frame header marking each frame as belonging to a specific VLAN. Normally this trunk will carry traffic for each VLAN present on the switch.

However routers only have one Ethernet MAC address per port, and normally each port is part of a single IP subnet. This means a solution must be found to allow the router to route traffic on a single port as if it were part of multiple separate IP subnets, and this is where subinterfaces come in.

As an example of what a typical subinterface setup might look like on a cisco router, a single ethernet port such as FastEthernet0/0 will be subdivided into Fa0/0.1 , Fa0/0.2 , Fa0/0.3 and so on, each one performing as if it were a separate interface. Each of these subinterfaces corresponds to a single VLAN on the switch.

Non Broadcast Multiple Access

In an NBMA network, or packets are not passed between circuits. This presents problems for traffic using these packets such as routing updates. To solve this problem, subinterfaces are sometimes used to allow for each logical circuit in a physical connection to be considered as its own interface, thus allowing for routing updates and other non-unicast traffic to be passed between them, which is otherwise not possible.