IS-IS Multitopology Failure

C.

IS-IS (Intermediate System to Intermediate System) is a link-state hierarchical routing protocol that is widely used in service provider networks. It supports multiple topologies within a single routing domain, which allows for better scalability, security, and traffic engineering.

The question is asking about the reason why a network team failed to implement IS-IS multitopology. Among the options provided, the most likely reason for this failure is option A: "The routing process supported Level 1 only."

IS-IS has three levels of hierarchy: Level 1 (L1), Level 2 (L2), and Level 1-2 (L1/L2). Each level is responsible for a different scope of routing information. Level 1 routers are responsible for routing within a single area, while Level 2 routers are responsible for inter-area routing. L1/L2 routers are dual-homed and perform both L1 and L2 routing functions.

Multitopology in IS-IS allows for multiple instances of L1/L2 routing within a single routing domain. Each instance can have its own topology, which enables the network to be segmented for different applications or customers. However, to use multitopology, the routing process must support L1/L2. If the routing process supports L1 only, then multitopology cannot be implemented.

Option B, "The router did not support VRFs," is not related to IS-IS multitopology. VRFs (Virtual Routing and Forwarding) are a way to create multiple virtual routing tables on a single physical router. While VRFs can be used in conjunction with IS-IS, they are not a requirement for multitopology.

Option C, "The routing process did not support extended metrics," is also not directly related to IS-IS multitopology. Extended metrics allow for more granular path selection in IS-IS by taking into account additional factors such as link bandwidth, delay, and reliability. However, even without extended metrics, IS-IS can still perform basic routing functions.

Option D, "The router did not have Cisco Discovery Protocol and Cisco Express Forwarding disabled," is also not a direct cause for failing to implement IS-IS multitopology. Cisco Discovery Protocol (CDP) is a proprietary protocol used by Cisco devices to exchange information about other Cisco devices on the same network. While it is recommended to disable CDP on non- Cisco devices to improve security, it is not a requirement for IS-IS multitopology. Cisco Express Forwarding (CEF) is a fast switching technology used by Cisco routers. While CEF can improve routing performance, it is not a requirement for IS-IS multitopology.

In conclusion, the most likely reason for the network team's failure to implement IS-IS multitopology is that the routing process supported L1 only, which is option A.