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Which three statements about IS-IS in a multi-area network are correct? (Choose three.)
Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It operates in two levels, Level 1 (L1) and Level 2 (L2), and supports hierarchical routing within a multi-area network.
Let's analyze each statement to determine its correctness in the context of IS-IS multi-area networks.
1. **Statement A: Internal L1 PDUs are flooded to the local area's L2 routers.**
- This statement is correct. L1 PDUs (Protocol Data Units) are flooded within the L1 area and also to the L2 routers that are present in the same area. These L2 routers act as the boundary routers that connect the local L1 area to other L1 areas via L2.
2. **Statement B: External L2 PDUs are flooded to all L2 routers in other areas.**
- This statement is correct. L2 PDUs are flooded throughout the entire L2 backbone, which includes all L2 routers in different areas. This ensures that inter-area routing information is shared across the network.
3. **Statement C: Internal L1 PDUs are flooded to all L1 routers in other areas.**
- This statement is incorrect. Internal L1 PDUs are only flooded within the local L1 area. They do not cross L1 area boundaries; inter-area communication is handled by L2 routers.
4. **Statement D: Internal L1 PDUs are only flooded to the local area's L1 routers.**
- This statement is correct. Internal L1 PDUs are indeed only flooded within their local L1 area, and do not go beyond it.
5. **Statement E: External L2 PDUs are only flooded to the local area's L2 routers.**
- This statement is incorrect. External L2 PDUs are flooded to all L2 routers throughout the IS-IS network, not just to those in the local area. This allows L2 routers to maintain a complete map of the network's topology.
**Conclusion**:
Given the analysis, the correct answers are:
**A. Internal L1 PDUs are flooded to the local area's L2 routers.**
**B. External L2 PDUs are flooded to all L2 routers in other areas.**
**D. Internal L1 PDUs are only flooded to the local area's L1 routers.**
**Reference**:
- Juniper Networks Documentation on IS-IS: [IS-IS Overview](https://www.juniper.net/documentation/en_US/junos/topics/concept/is-is-routing-overview.html)
- RFC 1195, Use of OSI IS-IS for Routing in TCP/IP and Dual Environments: [RFC 1195](https://tools.ietf.org/html/rfc1195) which details the operation of IS-IS in multi-area networks.