IS-IS Fundamentals and Protocol Operation
What the exam tests
IS-IS is a link-state interior gateway protocol standardised in ISO 10589 for CLNP and extended by RFC 1195 to carry IP. The NRS II composite expects you to know its PDU families, its two-level hierarchy, and the operational differences from OSPF that appear in Nokia SR OS outputs.
Core operation
- IS-IS runs directly over Layer 2; IIH, LSP and SNP PDUs are not encapsulated in IP.
- Three PDU families: IIH (hellos), LSP (link-state PDUs) and SNP (CSNP for database description, PSNP for acknowledgement and requests).
- Level 1 is intra-area; Level 2 is the backbone and carries inter-area reachability.
- On broadcast links a DIS is elected by priority, then highest MAC; there is no backup DIS and the DIS preempts.
- The DIS originates a pseudonode LSP so the LAN appears as one node in SPF.
- Integrated IS-IS carries IPv4 and IPv6 reachability in TLVs rather than running separate protocol instances.
Worked example
Three routers share a LAN: R1 priority 64 MAC 0000.0000.0001, R2 priority 64 MAC 0000.0000.0002, R3 priority 100. R3 is DIS because priority is compared first. If R3 fails, R1 becomes DIS because the MAC tie-break is highest, not lowest. The DIS advertises a pseudonode LSP whose LSP ID uses a non-zero pseudonode ID, and each router advertises a link to that pseudonode.
Traps
Assuming hellos are IP packets; expecting a BDR; forgetting that DIS election is preemptive; treating L1 and L2 hellos as interchangeable.