In the intricate world of Internet Service Provider (ISP) environments, the IS-IS protocol stands as a stalwart Interior Gateway Protocol (IGP). Its nuanced configuration, troubleshooting intricacies, and dynamic functionality make it a cornerstone of modern networking. In this exploration, we delve into the depths of IS-IS, shedding light on its area types, configuration nuances, and troubleshooting methodologies through real-world scenarios and configurations.
Understanding IS-IS
Integrated IS-IS, often referred to simply as IS-IS, forms the backbone of routing in ISP environments, seamlessly integrating with the Internet Protocol (IP). Its adaptability and extensibility stem from its ingenious use of TLVs (Type-Length-Value), allowing for the seamless integration of new features into the protocol.
Deciphering IS-IS Areas
Unlike OSPF, where any router interface can be assigned to a specific area, IS-IS operates on a different paradigm. Here, each router inherently belongs to an area, reflecting its origins in routing the Connectionless Network Protocol (CLNP). IS-IS operates on two hierarchical levels: Level 1 and Level 2.
- Level 1: Comparable to OSPF’s intra-area routing, Level 1 routers form the foundation of IS-IS routing within an area.
- Level 2: Mirroring OSPF’s backbone Area 0, Level 2 routers facilitate routing between different areas, serving as the backbone of IS-IS routing.
Every Cisco router defaults to being a Level 1-2 (L1/L2) router, capable of participating in both Level 1 and Level 2 routing. Level 1 routers can establish adjacencies with both Level 1 and Level 1-2 routers, while Level 2 routers exclusively interact with other Level 2 or Level 1-2 routers.
Delving into IS-IS Router Types
Understanding the nuances of IS-IS router types is pivotal to grasping its operational dynamics.
- IS-IS Level 1 (L1) Router: These routers possess comprehensive link state information within their area and rely on Level 2 routers to route packets beyond their area boundaries.
- IS-IS Level 1-2 (L1/L2) Router: Operating at the intersection of Level 1 and Level 2 routing, these routers maintain separate link state databases for each level. Similar to OSPF’s Area Border Routers (ABRs), they facilitate inter-area routing and communicate using both Level 1 and Level 2 hellos.
- IS-IS Level 2 (L2) Router: Solely focused on Level 2 routing, these routers handle both intra-area and inter-area routing, analogous to OSPF’s backbone routers in Area 0.
Potential Adjacency Capabilities
| Global Level | Interface Level | Potential Adjacency |
| Level 1/2 | Level 1/2 | Level 1 and/or level 2 |
| Level 1/2 | Level 1 | Level 1 only |
| Level 1/2 | Level 2 | Level 2 only |
| Level 2 | Level 1/2 | Level 2 only |
| Level 2 | Level 2 | Level 2 only |
| Level 2 | Level 1 | None |
| Level 1 | Level 1/2 | Level 1 only |
| Level 1 | Level 2 | None |
| Level 1 | Level 1 | Level 1 only |
IS-IS Adjacency Table
| Router Type | L1 | L1/L2 | L2 |
| L1 | L1 Adjacency if Area Id Matches, else no Adjacency | L1 Adjacency if Area Id Matches, else no Adjacency | No Adjacency |
| L1/L2 | L1 Adjacency if Area Id Matches, else no Adjacency | L1 and L2 Adjacency if Area id Matches , else only L2 Adjacency | L2 Adjacency , Area Id doesn’t matter |
| L2 | No Adjacency | L2 Adjacency , Area Id doesn’t matter | L2 Adjacency , Area Id doesn’t matter |
| MTU | If an IS-IS router receives an ISIS hello packet with a higher Maximum Transmission Unit (MTU) than it can support on the interface, it will discard the hello packet, resulting in the adjacency failing to establish. It’s essential for optimal functionality that the MTU is consistent on both ends of the connection. |
| Circuit-Type | The IS-IS protocol allows for the configuration of an attribute on interfaces, specifying whether L1 or L2 hellos are transmitted. A router configured as L1/L2 can selectively send L1-only hellos on one interface and L2-only hellos on another. However, if an L1/L2 router attempts to establish a peering with an L1-only router while the interface is configured with “isis circuit-type level-2,” it will only send L2 hellos, preventing the adjacency with the L1 router from forming. To ensure successful peering, routers must send compatible types of hellos. |
| Authentication | IS-IS offers the capability to authenticate both hellos and Link State Protocol Data Units (LSP) independently. In cases where hellos are authenticated successfully but LSP authentication fails, the adjacency will establish, but updates will not be exchanged. It’s crucial that if authentication is configured for IS-IS hellos or PDUs (Protocol Data Units), it must match on both ends to ensure proper functionality. |
| Capability TLV | If an IS-IS Router encounters a Capability TLV that it doesn’t support from another IS-IS Router, it will simply ignore the TLV. However, instances of capability mismatch may arise, leading to one router entering the INIT state while the other discards packets, resulting in the failure to form an adjacency. It’s advisable for the Capability TLV to match for successful adjacency establishment. While delving into the specifics of the Capability TLV is beyond the scope of this document, ensuring compatibility is key to seamless router communication. |
| Network Type | In IS-IS, there are two network types: Broadcast and Point-to-Point. By default, the network type is Broadcast. If one end is configured with “isis network point-to-point” while the other end remains at the default network type, hellos will be discarded, and the adjacency will fail to establish. It’s imperative for the network type to match on both ends for successful adjacency. |
| Hellos | Hello timers need not match for the adjacency to come up. |
Navigating IS-IS Adjacency States
IS-IS adjacency states provide insight into the operational status of router connections.
- Down: The initial state indicating no communication between neighboring routers.
- Initializing: Signifies successful receipt of hellos from a neighbor, yet awaiting confirmation of mutual communication.
- Up: Confirms bi-directional communication between neighboring routers, indicating a stable adjacency.
Conclusion
In the labyrinthine realm of ISP networking, IS-IS protocol emerges as a robust and adaptable solution. Its unique approach to areas, router types, and adjacency management underscores its resilience in complex environments. By unraveling the mysteries of IS-IS, network engineers can harness its power to build scalable and efficient networks that form the backbone of modern connectivity.
