You are configuring an IS-IS IGP network and do not see the IS-IS adjacencies established. In this scenario, what are two reasons for this problem? (Choose two.)
A.
MTU is not at least 1492 bytes.
B.
IP subnets are not a /30 address.
C.
The Level 2 routers have mismatched areas.
D.
The Io0 interface is not included as an IS-IS interface.
AD are correct - ac89l explained it well.
Also, B and C would be incorrect because: IS-IS adjacencies don't rely on IP protocol, you don't even need an IP configuration on links used for forming an adjancency.
Also, the one of the points of Level-2 routers is to form an adjacency between different areas.
chatGPT:
The two reasons for the IS-IS adjacencies not being established could be:
A. MTU is not at least 1492 bytes.
D. The Io0 interface is not included as an IS-IS interface.
Explanation:
A. IS-IS may have issues establishing adjacencies if the Maximum Transmission Unit (MTU) is not set correctly. It is recommended to set the MTU to at least 1492 bytes for IS-IS.
D. The Loopback 0 (Io0) interface should typically be included as an IS-IS interface. Loopback interfaces are often used as the source for IS-IS routing updates, and omitting them could prevent adjacencies from forming.
So, options A and D are the correct reasons for the problem.
I correct my statement. Lower MTU than 1492, IS-IS works "sometimes" due to the PDU being fragmented for the bigger PDU with CSNPs or PSNPs. 'A' is definitely the best choice here.
I crossed checked this with a lab. IS-IS does not have strick requirements for mtu. MTU size is important for overall network performance, it is not a direct cause of IS-IS adjacency issues in this context.
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