Correct Answer: C
To determine R3's role (Level-1, Level-2, or Level-1-2) based on the provided output of the display ipv6 routing-table protocol isis command on R3, we need to analyze the IS-IS routing information and understand IS-IS hierarchy and behavior, particularly in the context of IPv6 routing. Let's break it down step by step:
Understanding the IS-IS Levels and Hierarchy:
IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol that supports a two-level hierarchy:
Level-1: Routers within the same area, responsible for intra-area routing. Level-1 routers only maintain routing information for their area and rely on Level-1-2 routers to reach other areas.
Level-2: Routers that form the backbone, connecting different areas for inter-area routing. Level-2 routers maintain routing information across areas.
Level-1-2: Routers that operate both at Level-1 (within their area) and Level-2 (to connect to other areas).
These routers act as boundary routers between Level-1 areas and the Level-2 backbone.
A router's role (Level-1, Level-2, or Level-1-2) determines which types of adjacencies it can form and what routing information it advertises or receives.
Analyzing the Output:
The output shows R3's IPv6 routing table with routes learned via IS-IS, specifically:
Destination: 2082:EDFC:DDCC::B824:0/127, Protocol: ISIS-L1
This route is learned via IS-IS Level-1 (ISIS-L1), indicating that R3 has a Level-1 adjacency and is receiving or advertising routes within its area.
The next hop (FE80::2E0:FCFF:FE45:6A3E) and interface (GigabitEthernet0/0/0) suggest this route is intra- area.
Destination: 2082:EDFC:DDCC::B891:0/127, Protocol: ISIS-L2
This route is learned via IS-IS Level-2 (ISIS-L2), indicating that R3 has a Level-2 adjacency and is receiving or advertising routes for inter-area or backbone routing.
The next hop (FE80::2E0:FCFF:FE50:406F) and interface (GigabitEthernet0/0/1) suggest this route is for a different area or the backbone.
Both Level-1 and Level-2 routes are present in R3's routing table, which provides critical insight into R3's role.
Determining R3's Role:
Level-1 Routers:
Level-1 routers only participate in Level-1 adjacencies and maintain routing information for their area. They do not have Level-2 routes or adjacencies unless connected to a Level-1-2 router. Since R3 has a Level-2 route (ISIS-L2), it cannot be exclusively a Level-1 router.
Level-2 Routers:
Level-2 routers focus on inter-area routing and typically do not maintain Level-1 routes unless they are also configured as Level-1-2. Since R3 has a Level-1 route (ISIS-L1), it cannot be exclusively a Level-2 router.
Level-1-2 Routers:
Level-1-2 routers operate both at Level-1 (for intra-area routing) and Level-2 (for inter-area routing). They can form adjacencies with both Level-1 and Level-2 neighbors and maintain both types of routes. The presence of both ISIS-L1 and ISIS-L2 routes in R3's routing table indicates that R3 is participating in both Level-1 and Level-2 adjacencies, making it a Level-1-2 router.
Evaluating Each Option:
A). R3 must be a Level-2 device.
This is incorrect because R3 has ISIS-L1 routes, indicating it is also participating in Level-1 adjacencies, which Level-2-only devices do not do. Level-2 devices focus solely on inter-area routing and do not maintain Level-1 routes.
B). The device role of R3 cannot be determined.
This is incorrect because the presence of both ISIS-L1 and ISIS-L2 routes in the routing table clearly indicates R3's role. The combination of Level-1 and Level-2 routes confirms R3 is a Level-1-2 router.
C). R3 must be a Level-1-2 device.
This is correct. The routing table shows both ISIS-L1 (Level-1) and ISIS-L2 (Level-2) routes, meaning R3 is operating as both a Level-1 and Level-2 router, which defines a Level-1-2 device.
D). R3 must be a Level-1 device.
This is incorrect because R3 has ISIS-L2 routes, indicating it is also participating in Level-2 adjacencies, which Level-1-only devices do not do. Level-1 devices only maintain intra-area routes and rely on Level-1-2 routers for inter-area reachability.
Additional Considerations:
The Flags: D in the output typically indicates a directly connected route or a route downloaded to the forwarding table, but it does not affect the determination of R3's IS-IS level.
The preference (15) and cost (20) are standard for IS-IS routes and do not impact the level determination.
In Huawei's IS-IS implementation, routers can be configured explicitly as Level-1, Level-2, or Level-1-2 using commands like isis level-1-2 under the IS-IS process. The presence of both Level-1 and Level-2 routes confirms R3's configuration as Level-1-2.
Huawei HCIE-Datacom Context:
According to Huawei HCIE-Datacom documentation, IS-IS routers can operate at different levels, and the routing table's protocol type (ISIS-L1 or ISIS-L2) indicates the level of adjacency. A router with both Level-1 and Level-2 routes is necessarily a Level-1-2 device, as it participates in both intra-area and inter-area routing.
The display ipv6 routing-table protocol isis command on Huawei devices shows routes learned via IS-IS, and the protocol type (L1 or L2) directly reflects the router's role in the IS-IS hierarchy.
Conclusion:
Based on the presence of both ISIS-L1 and ISIS-L2 routes in R3's IPv6 routing table, R3 must be a Level-1-2 device, as it is participating in both Level-1 (intra-area) and Level-2 (inter-area) adjacencies. Therefore, the correct answer is C.
References to Huawei HCIE-Datacom Documents:
Huawei HCIE-Datacom V1.0 Training Material, Chapter on IS-IS Configuration and Hierarchy.
Huawei NE Series Router Configuration Guide, IS-IS Section (Level-1, Level-2, and Level-1-2 Operations).
Huawei Technical Whitepaper on IS-IS Protocol, discussing IPv6 Routing and Level Determination.
