IS-IS Levels, Area Boundaries and Inter-Area Routing
What the exam tests
This domain tests the roles of L1, L2 and L1/L2 routers, the ATT bit, route leaking and the requirement for a contiguous L2 backbone.
Core rules
- L1 routers stay inside one area and hold only L1 LSPs.
- L2 routers form the backbone and carry inter-area prefixes.
- L1/L2 routers attach an L1 area to the L2 backbone and set the ATT bit in their L1 LSP when they have usable L2 connectivity.
- L1 routers install a default route toward the nearest attached L1/L2 router.
- L2-to-L1 route leaking exports selected L2 prefixes into L1; it must be controlled to avoid scaling and loop problems.
- Summarization typically happens on L1/L2 routers at the area boundary.
Worked example
An L1/L2 router loses all L2 adjacencies but keeps its L1 adjacencies. It clears the ATT bit in its L1 LSP. L1 routers recompute and remove the default route pointing to it, so they must use another attached L1/L2 router. If no alternative exists, the L1 area becomes isolated from the backbone even though L1 adjacencies remain Up.
Traps
Assuming a discontiguous L2 backbone still works; leaving the ATT bit set without L2 connectivity; leaking all L2 routes into L1; forgetting that L1 areas cannot be non-contiguous across an L2 cloud.