Posts Tagged ‘rfc2328’

Sep
14

The feature we are going to talk about today may look a bit convoluted, but it demonstrates core OSPF behavior: combining link-state and distance-vector behaviors. The command capability transit was introduced in IOS 12.3T and is on by default. However, the description is rather confusing and does not explain the underlying mechanics. We are going to give an in-depth look at this feature now.

What is Transit Capability?

In short, this is a special property of a non-backbone area that allows this area to transport traffic for other areas (either zero or non-zero). Per the OSPF definition, a transit area is the area that has a virtual-link connecting two or more ABRs attached to this area. Thus, having a virtual-link provisioned across the area is the necessary thing to make the area transit. In fact, it’s just an alternate definition of a transit area. So the first thing we want to find out is what kind of mechanism is a virtual-link?

What are Virtual-links?

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