Level 1 inside an area, Level 2 between areas
Integrated IS-IS is a link-state routing protocol that can carry IPv4 and IPv6 reachability. Routers send IS-IS Hellos directly over the link and identify themselves with system IDs from their network entity titles (NETs).
Level 1 provides routing inside an area and requires a common area address for adjacency. Level 2 provides routing between areas and can form an adjacency across different area addresses. A Level 1–2 router participates in both levels and maintains a separate database for each.
Link State PDUs carry TLVs
IS-IS floods Link State Protocol Data Units (LSPs) within a level. Type-length-value (TLV) fields inside an LSP carry details such as neighboring systems and reachable prefixes. For example, TLV 135 carries extended IPv4 reachability and TLV 236 carries IPv6 reachability.
OSPF exchanges individual link-state advertisements with distinct LSA types. IS-IS packages reachability and topology fields inside LSPs and runs SPF separately for each level in which it participates.
Try it in a lab
- Connect two IS-IS-capable routers over a point-to-point link. Set their NETs to different area addresses with unique system IDs and enable only Level 2 on both ends.
- Verify a Level 2 adjacency and a route to a loopback advertised by the other router. On a platform using wide IPv4 metrics, inspect the remote LSP for the loopback in the extended IP reachability TLV.
- Change both routers to Level 1 only while keeping their areas different. Verify the adjacency fails to form and the learned route disappears. Give the routers a common area address and verify the Level 1 adjacency and route recover.