Part 06 – Layer 3 Network Routers VLANs Switches

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Part 06 – Layer 3 Network Routers VLANs Switches

In this section we make the leap into a more intelligent layer of the OSI model with the addition of IP addressing at layer 3. This introduction includes both the protocols that reside at layer 3, as well as the more complex – and expensive – layer 3 devices, most notably, routers.

As we saw in the previous section, there is only so much that can be accomplished towards reducing data collisions even with the move to layer 2 switches from old fashioned hubs. The issue of multiple broadcast domains and the traffic congestion they cause can be effectively dealt with by taking advantage of IP addressing at layer 3.

We’ll examine an enhanced feature available on some layer 2 switches known as VLANs (Virtual LANs). VLANs provide isolated broadcast domains on switches supporting this feature. Just keep in mind that even with this feature, these are still layer 2 devices natively and as such, don’t possess the ability to understand IP addressing.

For this level of addressing, we need to look to routers, which are layer 3 devices. We point out in this section that routers come with a price both in the form of higher cost and more complex configurations. We make the important note that routers are required in order to interconnect VLANs since this can only be accomplished via IP addressing.

Finally, we emphasize that these subtle distinctions are important to know for the CISSP exam.

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