ISIS Levels and Areas
IS-IS routing protocol is extensively used as Interior Gateway Protocol (IGP) in Service Provider (SP) environment. This document helps you gain information regarding IS-IS level types, configuration and troubleshooting. In Cisco world, Integrated IS-IS is deployed, meaning ISIS is used to route IP. We will discuss all the concepts in this article with respect to Integrated ISIS. The strength of ISIS protocol is that it uses TLVs (Type-Length-Value) making IS-IS highly extensible protocol. As new features come in, they can be added to protocol using TLVs.
Concept of Area in ISIS
Coming back to OSPF protocol, the router’s interfaces are assigned to a particular area,for instance, on an ABR, one interface is part of area 0 and another interface could be part of a connected area. However, the concept of area in ISIS is different. In ISIS, a router belongs to an Area. This means that all interfaces of the router will be part of same area. The idea of this comes from the fact that ISIS was initially created to route Connection Less Network Protocol (CLNP) where the address belongs to a device (Router - NSAP address), whereas in Internet Protocol (IP) the address belongs to the particular interface.
IS-IS protocol has two levels or hierarchy, Level 1 and Level 2 explained later in this article. Level 1 corresponds OSPF intra-area routing whereas Level 2 corresponds with the OSPF backbone Area 0 routing. Level 2 areas join all the areas with the backbone area. Every Cisco router comes by default as Level 1-2 (L1/L2) router to allow for easy configuration and deployment.
A Level 1 router can become adjacent with the Level 1 and Level 1-2 (L1/L2) router. A Level 2 router can become adjacent with Level 2 or Level 1-2 (L1/L2) router. There is no adjacency between L1 only and L2 only router.
IS-IS protocol has two levels or hierarchy, Level 1 and Level 2 explained later in this article. Level 1 corresponds OSPF intra-area routing whereas Level 2 corresponds with the OSPF backbone Area 0 routing. Level 2 areas join all the areas with the backbone area. Every Cisco router comes by default as Level 1-2 (L1/L2) router to allow for easy configuration and deployment.
A Level 1 router can become adjacent with the Level 1 and Level 1-2 (L1/L2) router. A Level 2 router can become adjacent with Level 2 or Level 1-2 (L1/L2) router. There is no adjacency between L1 only and L2 only router.
ISIS has a 2 layer hierarchy
- Level-2 (the backbone)
- Level-1 (the areas)
Level of routing
- Level 0 : Between an End System (a host or a PC) and IS (a Router).
- Level 1 : IS to IS in the same area.
- Level 2 : IS to IS in different areas in the same AS.
- Level 3 : IS to IS in different AS or routing domain. Like routing through ASBR in OSPF.
An ISIS router can be either of these
- Level-1 (L1) router.
- Level-2 (L2) router.
- Level-1-2 (L1/L2) router(default type - Cisco).
Level-1 router
- Has neighbors only in the same area.
- Has a level-1 LSDB with all routing information for that area.
Level-1-2 router
- May have neighbors in any area.
- Has two separate LSDBs: level-1 LSDB & level-2 LSDB.
- By default, a router becomes L1/L2 router unless specifically changed the role to L1-only or L2-only.
Level-2 router(kind of a backbone area)
- May have neighbors in the same or other areas.
- Has a Level-2 LSDB with all routing information about inter-area.
ISIS Adjacency States
There are three adjacency states in ISIS as described below:- Down - This is the initial state. Its means that no hellos have been received from the neighbor.
- Initializing - In this state, the local router has successfully received hellos from the neighboring router, however, it is not sure that the neighboring router has also successfully received local router’s hellos.
- Up - Now it’s confirmed that neighboring router is receiving local router’s hellos.
ISIS Adjacency Formation - Important factors
MTU
If a router running ISIS receives an ISIS hello packet with higher MTU than it can support (show interface gi1/0/0 | include MTU) it discards the hello packet and the adjacency does not come up. It is advisable to keep MTU same on both ends of links/interfaces of the routers running ISIS.Circuit-Type
This attribute is configured at interface level (isis circuit-type command) and defines what type of hellos i.e. L1 or L2 are sent on a particular interface. Default mode is L1/L2 if not is-type is defined using the command. A L1/L2 router can selectively send L1 only hellos on one interface to L1 peers and L2 only hellos on its other interface to the L2. If L1/L2 router is trying to peer with an L1 only router and L1/L2 interface is configured with “isis circuit-type level-2” it will only send L2 hellos out the interface and the adjacency with L1 router will not come up. Hence routers must send compatible type hellos.
Authentication
ISIS can be configured separately authenticate hellos and Link State Protocol Data Units (LSP). If hellos are authenticated correctly and LSP authentication fails, the adjacency will come up but updates will not be exchange. Therefore if authentication is configured for ISIS hellos or PDUs (Protocol Data Unit) must match on both the ends.
Capability TLV
If an IS-IS Router does not support the Capability TLV from the other IS-IS Router it silently ignores the TLV. However, there might be events due to capability mismatch when one router reaches INIT state whereas the other one discards the packets and doesn’t form adjacency. So as a general recommendation Capability TLV must match for successful adjacency formation. Discussing in depth details for Capability TLV is beyond the scope of this document.
ISIS Network Type
There are only two network types in ISIS unlike OSPF. Broadcast and Point-to-Point. Broadcast is default network type in ISIS. If one end is configured with “isis network point-to-point” and other end is default network type. The hellos will be discarded and adjacency will not come up. Hence network type must match on both the ends.
