Is-Is

IS-IS IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol used primarily in service provider networks. Like OSPF, it uses the Dijkstra SPF algorithm to compute shortest paths, but runs natively over CLNS rather than IP, making it protocol-agnostic and well-suited for multi-protocol environments. Terms ES (End System) - A host that originates and receives packets. ES-to-ES communication is host-to-host. IS (Intermediate System) - A router that forwards packets. IS-IS describes routing between intermediate systems. CLNS/CLNP - IS-IS runs natively over the Connectionless Network Service (CLNS) using CLNP, not IP. This is a key distinction from OSPF. NSAP (Network Service Access Point) - The addressing scheme IS-IS uses instead of IP addresses. NET (Network Entity Title) - The IS-IS address configured on a router. Format: Area ID . System ID . NSEL Example: 49.0001.1921.6800.1001.00 49.0001 — Area ID 1921.6800.1001 — System ID (6 bytes, often derived from an IP like 192.168.1.1) 00 — NSEL (always 00 for a router) System ID - 6-byte unique identifier for a router within an area (similar to OSPF Router ID). NSEL (N-Selector) - The last byte of a NET, always 00 for routers. L1 router - Routes only within its area; sends traffic to unknown destinations toward the nearest L1/L2 router. L2 router - Routes between areas and toward other ASes. L1/L2 router - Does both; this is the Junos default. Link-State Database Runs the Dijkstra SPF algorithm. L1 and L2 maintain separate LSDBs — SPF is run independently for each level. Each router originates its own LSP and floods it throughout its level. LSDB synchronization is handled by CSNPs (full sync) and PSNPs (fill gaps). IS-IS Protocol Data Units (PDUs) IIH (IS-IS Hello) - Used to discover neighbors and maintain adjacencies. Contains the router’s identity, capabilities, and configured area. L1 LAN IIH: Sent by Level 1 routers on multi-access networks (like Ethernet). L2 LAN IIH: Sent by Level 2 routers on multi-access networks. P2P IIH: A single format used for point-to-point links, regardless of level. LSP (Link State PDU) - Carries the actual routing information, including connected neighbors, configured prefixes, and metric costs. Each LSP has a sequence number, checksum, and remaining lifetime. L1 LSP: Contains routing information for the local area. L2 LSP: Contains backbone routing information. CSNP (Complete Sequence Number PDU) - Contains a complete list of all LSPs in a router’s LSDB. Used to ensure every router in the area has a consistent view of the network. L1 CSNP: Summarizes the Level 1 LSDB. L2 CSNP: Summarizes the Level 2 LSDB. On LAN segments, the DIS sends these periodically. On point-to-point links, they are typically sent only when the link first comes up. PSNP (Partial Sequence Number PDU) - Used to request missing LSPs or acknowledge receipt of specific LSPs. Unlike CSNPs, they only reference a subset of LSPs. L1 PSNP / L2 PSNP: Used to fill gaps after a CSNP reveals a missing LSP, or as an explicit ACK on point-to-point links. Type, Length, Value (TLVs) TLVs are the data structures embedded inside LSPs that carry routing information. Key TLVs to know for JNCIS-SP: ...