Level 1 (L1) routers in the IS-IS (Intermediate System to Intermediate System) routing protocol play a vital role in the hierarchical structure of the IS-IS routing domain. They are also commonly referred to as “intra-area” routers. Here’s a deeper dive into L1 routers:
Functionality: L1 routers maintain and share routing information only about their own IS-IS area. They do not have a complete view of the entire IS-IS domain. Instead, they are knowledgeable about the topology of their own area and know how to reach L2 routers (inter-area routers) within their area to forward traffic destined for other areas.
Link-State Database (LSDB): L1 routers maintain a Link-State Database that contains link-state information about other routers within the same area. This database is used to compute the best path to destinations within the area using the Shortest Path First (SPF) algorithm.
Routing Updates: L1 routers exchange link-state packets (LSPs) with other L1 routers within the same area. This ensures that all L1 routers in the area have a consistent view of the area’s topology.
Inter-Area Traffic: When an L1 router has data destined for a network outside its area, it forwards that data to the nearest L2 router. The L2 router then takes responsibility for routing the data to its correct destination outside the area.
Neighbor Discovery and Adjacency Formation: L1 routers discover neighbors and form adjacencies with other L1 routers in the same area using IS-IS Hello packets. These adjacencies are crucial for the exchange of LSPs.
L1 Default Route: Typically, L1 routers are configured with a default route pointing to the nearest L2 router. This ensures that any traffic not destined for the local area is sent towards an L2 router for further routing decisions.
Benefits:
- Scalability: By limiting the scope of L1 routers to their own area, the size of the LSDB is reduced, which in turn limits the computational overhead. This makes IS-IS scalable for large networks.
- Reduced Routing Traffic: Since L1 routers only exchange routing updates within their area, there’s a reduction in the amount of routing traffic, conserving bandwidth.
Drawbacks:
- Limited Topology View: L1 routers don’t have a view of the entire network’s topology. They rely on L2 routers to route traffic destined for other areas.
In a hierarchical IS-IS design, L1 routers effectively help segment the network, making routing more efficient and scalable. This hierarchical approach is especially beneficial for large networks where a flat routing structure would be inefficient.