Case Study: The Single Area Option
It is possible to put this entire network in a single IS-IS area using L2 routing only. The advantage to this is that all routing in the network is optimal. The disadvantage is that you can't summarize any place in the network.
There are three primary areas that you need to consider when choosing the single area option: the HQ VLANs and the common services areas (which have a large number of parallel links with hosts connected to them), and the dial-in clients, or other similar area, where the topology will change regularly.
HQ VLANs and Common Services Areas
The biggest issue with the HQ VLANs and common services areas is the multiple parallel LANs/VLANs. You don't necessarily want these paths to become transit ...
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