Exam 300-420 topic 1 question 15 discussion

You have an IS-IS router that is performing both L1 and L2 routing and has both L1 and L2 neighbors. How would you optimize the router's operation to conserve bandwidth and router resources? Configure each interface as either L1 or L2 circuit type, depending on the type of adjacency needed out that interface. The command to do this is, at the interface configuration mode, isis circuit-type [level-1 | level-1-2 | level-2-only]. This prevents unnecessary hellos from being sent out interfaces, which uses bandwidth and router resources. https://www.ciscopress.com/articles/article.asp?p=101756

"the router is configured as L1/L2 and has L1 and L2 neighbors" - the router has to stay L1/L2 but the circuit-type can be set on each interface based on the type of neighbor (L1 or L2 router), and that will decrease the load. If we change the router type to either L1 or L2, some neighborship will be lost, depending on the option chosen.

I was finally able to find the answer We all know that answers A and B make no sense. C: This answer is incorrect, it states that the router is L1/L2 and it's got L1 Neighbor and an L2 Neighbor, If you set the router to L1 it will make an adjacency with L1 but not L2, L1 Routers will become adjacent with L1 and L1/L2s but not with L2s only: https://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/200293-IS-IS-Adjacency-and-Area-Types.html The Answer is D: You can configure an interface as L1 or L2, this will reduce the amount of Hellos it's sent therefor improves performance.

I feel the answer is D. Answer C would actually be the best answer, because running 2 databases would consume the most resources. However this is making the assumption that we have the option of changing this particular router to a Level 1, or Level 2. The question doesn't state that this is an option. If the question stated, "What is the best possible way to cut down on router resources.." and left it at that, I would be inclined to say C is the answer. However since it doesn't, and we have to work with that we have (A router running both L1/L2), answer D is the best choice given the circumstances. In a real world scenario, we would apply option D. If that didn't fix the issue we would resort to option C.

https://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/command/reference/irs_book/irs_is1.html "You have an IS-IS router that is performing both L1 and L2 routing and has both L1 and L2 neighbors. How would you optimize the router's operation to conserve bandwidth and router resources? Configure each interface as either L1 or L2 circuit type, depending on the type of adjacency needed out that interface. The command to do this is, at the interface configuration mode, isis circuit-type [level-1 | level-1-2 | level-2-only]. This prevents unnecessary hellos from being sent out interfaces, which uses bandwidth and router resources."

You configure the router as Level 1, Level 2 or Level 1/2. Not the interface.

D is correct

To solve the acknowledgment problem and to reduce the size of the link-state database, we use a special mechanism. When IS-IS routers become neighbors, they also do an election to decide who becomes the DIS (Designated IS).

One of the few questions for this exam which I can't find the answer. Neither of the answers make sense for me.