In the context of the IS-IS (Intermediate System to Intermediate System) routing protocol, L1 (Level 1) and L2 (Level 2) denote hierarchical levels within the IS-IS domain. Here’s a breakdown of L1/L2 routers:

L1 (Level 1) Routers:

  1. Functionality: L1 routers function within an IS-IS area and are responsible for intra-area routing. They don’t have knowledge about areas outside their own.
  2. Link-State Database (LSDB): They maintain a Link-State Database containing information only about routers within their own area.
  3. Routing Updates: L1 routers exchange link-state packets (LSPs) with other L1 routers in the same area.
  4. Neighborship: They form neighbor relationships with other L1 routers in the same area and potentially with L1/L2 routers.
  5. Default Route: Typically, L1 routers will have a default route pointing to an L1/L2 or L2 router, directing traffic destined for other areas.

L2 (Level 2) Routers:

  1. Functionality: L2 routers function as the backbone of the IS-IS domain, maintaining inter-area routing information. They connect different IS-IS areas.
  2. LSDB: L2 routers have an LSDB containing link-state information about other L2 routers and the interconnections between areas.
  3. Routing Updates: They exchange LSPs with other L2 routers, encompassing the entire IS-IS domain.
  4. Neighborship: L2 routers form neighbor relationships with other L2 routers, and potentially with L1/L2 routers, but not with pure L1 routers outside their area.

L1/L2 (Level 1/Level 2) Routers:

  1. Functionality: These are routers that function at both L1 and L2 levels. They’re essentially “border” or “boundary” routers between an IS-IS area and the backbone.
  2. LSDB: They maintain two separate LSDBs – one for L1 and another for L2.
  3. Routing Updates: They exchange L1 LSPs with L1 routers in their area and L2 LSPs with L2 routers in the domain.
  4. Neighborship: They can form neighbor relationships with both L1 and L2 routers.
  5. Transition: They play a crucial role in transferring traffic from an L1 area to another area via the L2 backbone.

In a nutshell, the hierarchical structure of L1 and L2 routers in IS-IS helps in efficient routing and scalability. L1/L2 routers bridge the gap between the intra-area (L1) and inter-area (L2) domains, facilitating smooth communication within the entire IS-IS network.