10.2 IS-IS Levels, Areas and Default Routing

3. Hierarchical Routing Levels: Level 1 vs. Level 2

IS-IS implements a two-tier routing hierarchy designed to partition large networks and contain Dijkstra Shortest Path First (SPF) calculations:

+---------------------------------------------------------------------------------------------------+
| TWO-TIER IS-IS HIERARCHICAL TOPOLOGY                                                              |
|                                                                                                   |
|   +--------------------------+                      +--------------------------+                  |
|   |         AREA 49.0001     |                      |         AREA 49.0002     |                  |
|   |   [R1 (L1)]----[R2 (L1)] |                      |   [R5 (L1)]----[R6 (L1)] |                  |
|   |        \        /        |                      |        \        /        |                  |
|   |         [R3 (L1/L2)]-----+                      +-----[R4 (L1/L2)]         |                  |
|   +------------|-------------+                      +------------|-------------+                  |
|                |                                                 |                                |
|                +=================[ LEVEL 2 BACKBONE ]============+                                |
|                                (Contiguous L2 Path)                                               |
+---------------------------------------------------------------------------------------------------+

Level 1 (L1) Intra-Area Routing

• Routers running Level 1 maintain an identical Level 1 Link-State Database (L1 LSDB) detailing all routers, links, and prefixes inside their local area.
• Two routers can only establish a Level 1 adjacency if they share at least one area address in their configured NETs.
• Level 1 routers have no topological visibility outside their local area. To forward traffic destined for external areas, an L1 router forwards packets toward the nearest Level 1/Level 2 router that sets the Attached (ATT) bit in its Level 1 LSP, establishing an automatic default route (0.0.0.0/0).

Level 2 (L2) Inter-Area Backbone Routing

• Level 2 routers form the backbone of the IS-IS network, maintaining an identical Level 2 Link-State Database (L2 LSDB) containing all inter-area reachability information.
• Level 2 routers can establish L2 adjacencies across different Area IDs. For example, a router with Area ID 49.0001 can form a direct Level 2 adjacency with a router configured with Area ID 49.0002.
• The Level 2 backbone must be contiguous. If the Level 2 backbone partitions into disconnected segments, inter-area routing between those segments will fail.

Level 1-2 (L1/L2) Dual Border Routers

• An L1/L2 router sits at the perimeter between a Level 1 area and the Level 2 backbone.
• It maintains two distinct, independent Link-State Databases: an L1 LSDB and an L2 LSDB.
• It executes Dijkstra's SPF algorithm twice: once for Level 1 intra-area routing and once for Level 2 inter-area routing.
• It automatically advertises local Level 1 routes into the Level 2 backbone, enabling full reachability to internal subnets across the enterprise core.
• When it can reach an area other than its own through Level 2, the L1/L2 router sets the ATT (Attached) bit in its locally generated Level 1 LSPs to notify internal L1 routers of its backbone connectivity.

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4. Architectural Comparison: IS-IS vs. OSPF

Understanding IS-IS is greatly simplified by contrasting its structural mechanics with OSPF:

Critical Junos Operational Default

In Junos OS, when you enable IS-IS under [edit protocols isis], each explicitly enabled IS-IS interface defaults to operating in both Level 1 and Level 2 modes. Unless you explicitly disable an unneeded level using set protocols isis interface <interface-name> level 1 disable or level 2 disable, the router can participate at both levels. LANs use separate level-specific Hellos; a point-to-point circuit uses a common P2P Hello and negotiates the supported levels.

Worked Example: Choosing an Area Exit

Suppose an L1-only access router has two paths to attached L1/L2 routers. The shortest L1 path to BORDER-EAST costs 20; the path to BORDER-WEST costs 35. Both border routers advertise attached status. With no more-specific matching route installed, the access router sends an off-area packet toward BORDER-EAST because 20 is the lower local SPF cost. The calculation does not compare the complete remote path beyond each border. A nearer exit can therefore produce a longer end-to-end journey than a farther exit.

Now suppose the east border loses its only usable route to another area. Its local access-facing interface can remain up even though it no longer qualifies to advertise attachment. After the updated LSP is received and SPF converges, the access router uses the remaining attached west border. If neither border advertises attachment, investigate the missing default rather than inventing a static route that hides a backbone failure. A successful local adjacency proves neighbor communication, not off-area reachability.

Separate Level Configuration from Reachability Evidence

On the access router, disable Level 2 globally when the intended design is L1-only. On a border router, retain both levels globally, then restrict the access-facing circuit to L1 and the core-facing circuit to L2:

# Access router: retain Level 1 only
set protocols isis level 2 disable

# Border router: these statements apply to different interfaces
set protocols isis interface ge-0/0/0.0 level 2 disable
set protocols isis interface ge-0/0/1.0 level 1 disable

These snippets assume unique valid NETs, configured protocol interfaces and family iso on the participating links. An interface-level setting cannot restore a level disabled globally. Check show configuration protocols isis before interpreting a missing adjacency; the peer may have a different circuit-level policy even when both routers support both levels.

For verification, inspect show isis adjacency for the expected level and neighbor, then inspect show isis database detail for the border's L1 LSP and attached status. Finally, use show route 0.0.0.0/0 exact to inspect the installed default and its next hop. Compare these three layers of evidence instead of inferring an active route from the presence of a configured protocol statement.

An area address is not an OSPF-style per-interface assignment. Junos can support multiple NET area addresses, including during transitions; adjacent L1 routers need an area address in common. That exception does not turn each IS-IS interface into an independently assigned OSPF area. In ordinary steady-state diagrams, the L1 area boundary remains between routers connected through Level 2.

Official references checked 2026-10-18: IS-IS hierarchy and attached defaults, Level configuration and verification.