Interarea Tunnels
Up until now, you've seen TE tunnels that must start and end in the same area (or level, rather than area, if you're an IS-IS fan.) In fact, in older versions of Cisco IOS Software, a router was limited to a single TE database. This meant that not only could you not have a TE tunnel that spanned multiple areas, you couldn't have a router that was connected to multiple areas and had TE tunnels in each area independently. However, this restriction can be overcome using interarea tunnels.
IGP Terminology
Before diving into how interarea TE tunnels work, it's important to get the terminology straight. In OSPF, an area is a set of routers that share a common SPF tree. IS-IS, however, uses the term level to mean the same thing. Because ...
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