What is a common characteristic of IS-IS routing protocol?

The IS-IS (Intermediate System to Intermediate System) routing protocol is a link-state routing protocol used primarily in large networks and service provider environments. One of its defining characteristics is that it focuses on link state information to make routing decisions, which allows routers to have a complete view of the network topology.

While option B might seem reasonable at first glance, IS-IS does not have a default metric cost consistently set to 10. Instead, it uses a more dynamic metric system where the cost can vary based on the configuration and the settings of the network interfaces. The correct answer focuses on how IS-IS operates fundamentally.

In the context of the other options, IS-IS does indeed focus on link state, which is essential for its operation, allowing routers to effectively exchange information about the state of their links with other routers in the network. This characteristic of utilizing link-state information enables IS-IS to build a complete topology for optimal routing. This makes the protocol scalable and efficient in handling routing within extensive networks.

Therefore, a common characteristic of the IS-IS routing protocol is its focus on link state, which greatly contributes to its robust and scalable routing capabilities.