You are asked to deploy a large-scale IS-IS network with multiple areas.
In this scenario, which statement is true?
Click on the arrows to vote for the correct answer
A. B. C. D.D
IS-IS (Intermediate System to Intermediate System) is an interior gateway protocol (IGP) that is widely used in large-scale networks. It is a link-state routing protocol that operates on a hierarchical topology consisting of areas. Each area is identified by a unique Area ID, and routers within the same area share topology information.
IS-IS supports two levels of hierarchy: Level 1 and Level 2. Level 1 routers are responsible for routing packets within the same area, while Level 2 routers are responsible for routing packets between different areas. Level 2 routers are also known as backbone routers.
When a router is configured for Level 1, it advertises its connected networks and any prefixes that are redistributed into the IS-IS domain. However, a Level 1 router does not advertise external prefixes learned from other routing protocols or static routes.
When a router is configured for Level 2, it advertises all the prefixes learned from other routers in the same IS-IS domain, including external prefixes. Level 2 routers also advertise the existence of other Level 2 routers within the same area to facilitate backbone formation.
Based on the above information, we can conclude that the correct answer to the given question is B: A router configured for Level 1 and Level 2 informs all Level 2 routers in the area of external reachability.
The reason for this is that a router configured for both Level 1 and Level 2 will advertise all the prefixes learned from other routers in the same IS-IS domain, including external prefixes. Since Level 2 routers are responsible for routing packets between different areas, it is important for them to know about external reachability to be able to route packets to destinations outside the area.
It is also worth noting that a router configured for Level 2 only informs all Level 1 routers in the area of external reachability (Option C). However, this is not the best answer to the question since it does not mention Level 2 routers, which are also important in a large-scale IS-IS network with multiple areas.
Finally, a router configured for Level 1 only informs all Level 2 routers in the area of external reachability (Option A), and a router configured for Level 1 and Level 2 informs all Level 1 routers in the area of external reachability (Option D). However, these options are not the best answers to the question since they do not mention Level 2 routers, which are responsible for routing packets between different areas.