Summary

VLANs are designed to keep broadcasts within artificial limits, and this makes them a useful design tool. But nobody can expect that all of the data in one VLAN will remain there. Users will need to communicate with services and hosts on other VLANs, and that means going through a router.

Question: What do you get when you bridge two VLANs together?

Answer: A bigger VLAN!

So, to get from one VLAN to another, data needs to be forwarded across a router! Routers are needed to enable hosts on different networks to communicate, and also for inter-VLAN communication. The router in question can be either an external router or an internal route processor. Both are suitable and can do the task, but the advantages of internal processors are clear—cost, simplicity, and speed of link to the router fabric are all factors.

You can use both internal routers and external routers to configure an ISL/802.1Q. You can also use them for inter-VLAN configuration. Further, both ISL and 802.1Q are able to differentiate between VLANs at the router interface.