IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol used to move information efficiently within a computer network. Here is the detailed explanation about the statements:
* Option A: IS-IS uses areas and an autonomous system.
* This statement is correct. IS-IS organizes the network into a single autonomous system (AS), which can be divided into smaller groups called areas. Routing between areas is organized hierarchically. Each IS-IS network is treated as an autonomous system, and within that AS, multiple areas can be defined for efficient routing.
* Option B: Level 1/2 routers automatically inject a default route to the nearest Level 1 router.
* This statement is incorrect. Level 1/2 routers do not automatically inject default routes. Instead, they set the ATTACHED bit in their Level 1 link-state PDU when they can reach other areas, prompting Level 1 routers to use them as default gateways.
* Option C: Level 2 routers must share the same area address.
* This statement is incorrect. Level 2 routers operate across multiple areas and do not need to share the same area address. They are responsible for inter-area routing and thus work across different areas.
* Option D: Level 1 routers route traffic between autonomous systems.
* This statement is incorrect. Level 1 routers route traffic within a single area and forward inter-area traffic to Level 2 routers, which handle inter-area routing.
ConclusionGiven the above explanations, the correct statement about IS-IS is:
* A: IS-IS uses areas and an autonomous system.
This is based on the hierarchical structure of IS-IS routing where the network is divided into areas within a single autonomous system.
For further details, you can refer to the Juniper IS-IS Overview.