Cisco CCNP BSCI Exam Tutorial ISIS Router Types
Below is a MRR and PLR article in category Computers Technology -> subcategory Computer Certification.
Cisco CCNP / BSCI Exam Guide: Understanding ISIS Router Types
Mastering the details of ISIS is crucial for passing the BSCI exam and achieving your CCNP certification. While ISIS shares some similarities with OSPF, one key distinction is the presence of three different router types: Level 1 (L1), Level 2 (L2), and L1/L2.
Level 1 (L1) Routers
L1 routers operate within a single area and connect to other areas through L1/L2 routers. They use these L1/L2 routers as default gateways to access destinations in different areas, similar to how OSPF stub routers use ABRs.
L1 routers do not hold routing table entries for destinations outside their area; instead, they rely on an L1/L2 router to reach external networks. Additionally, L1 routers within the same area must synchronize their databases.
Level 2 (L2) Routers
L2 routers come into play during inter-area routing. Both L2 and L1/L2 routers handle this type of routing and keep their databases synchronized.
Hellos and Adjacencies
Like OSPF, ISIS routers use hello packets to form adjacencies. However, L1 routers send L1 hellos while L2 routers send L2 hellos. It's important to note that L1 and L2 routers on the same link will not form adjacencies.
L1/L2 Routers
Some routers can function as both L1 and L2 simultaneously, known as L1/L2 routers. These routers can have neighbors in different ISIS areas and maintain two distinct databases: one for L1 routes and another for L2 routes. Cisco routers running ISIS default to this L1/L2 configuration. They enable L1 routers to communicate with routers in other areas.
Stay tuned for the next section of this tutorial, where we'll dive deeper into ISIS hellos!
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