Level 2 (L2) routers, in the context of the IS-IS (Intermediate System to Intermediate System) routing protocol, function at a higher hierarchical level than Level 1 (L1) routers. Essentially, they operate as “inter-area” or “backbone” routers. Here’s an overview of L2 routers:

Functionality: L2 routers are responsible for maintaining and sharing routing information between different IS-IS areas. They have knowledge about the topology of all the areas and form the backbone of the IS-IS domain.

Link-State Database (LSDB): L2 routers maintain a Link-State Database that contains link-state information about other L2 routers and the interconnections between areas. This database allows them to determine the best path to reach other areas in the IS-IS domain.

Routing Updates: L2 routers exchange link-state packets (LSPs) with other L2 routers to maintain an updated and consistent view of the entire IS-IS domain.

Connecting Areas: L2 routers provide the critical interconnectivity between different IS-IS areas. They facilitate the routing of data between L1 routers in different areas.

Neighbor Discovery and Adjacency Formation: L2 routers discover neighbors and form adjacencies with both other L2 routers and L1 routers within the same area using IS-IS Hello packets.

L2 Default Route: L1 routers usually have a default route pointing to an L2 router. When an L1 router receives traffic destined for another area, it sends that traffic to an L2 router for inter-area routing.

Benefits:

  • Inter-Area Routing: L2 routers play a pivotal role in ensuring that traffic can flow between different IS-IS areas.
  • Reduced Complexity: By providing a hierarchical structure to routing, L2 routers help simplify the overall network topology and reduce the routing complexity for L1 routers.

Drawbacks:

  • Potential Bottlenecks: If not designed correctly, L2 routers can become bottlenecks in large networks. Redundancy and careful capacity planning are essential.
  • Larger LSDB: Since L2 routers need to maintain information about the entire IS-IS domain, their LSDB can become large in extensive networks. This might lead to increased computation times for SPF calculations.

In an IS-IS network, L2 routers act as the backbone, providing critical interconnectivity between areas. Their role is vital in ensuring that the network operates cohesively and efficiently. Proper design and redundancy are crucial to ensure that L2 routers provide the needed reliability and performance.