SPF Computation and Route Selection
What the exam tests
The exam tests Dijkstra operation per level, how L1 and L2 routes differ, and how Nokia SR OS chooses among competing routes.
Core rules
- IS-IS runs a separate SPF for Level 1 and Level 2.
- L1 SPF uses intra-area L1 LSPs plus a default route toward the nearest attached L1/L2 router, signalled by the ATT bit.
- L2 SPF uses L2 LSPs and inter-area prefixes; L1 prefixes are injected into L2 by L1/L2 routers.
- Route selection uses longest match, then administrative preference, then metric; equal-cost paths can be installed for ECMP.
- The up/down bit in extended reachability TLVs prevents inter-area prefixes from leaking back and creating loops.
Worked example
An L1 router has two exits: R2 with metric 10 and R3 with metric 20 to the L2 backbone. Both set the ATT bit, but SPF installs the default route via R2 because its cost is lower. If R2 loses its L2 adjacency, it clears the ATT bit and the L1 router recomputes the default via R3. The L1 router never learns the specific L2 prefixes unless route leaking is configured.
Traps
Expecting an L1 router to know specific inter-area routes by default; forgetting that narrow metrics cap at 63; ignoring the up/down bit; assuming the ATT bit is set even when the L1/L2 router has no L2 adjacency.