Dynamic Routing Protocol : Introduction to IS-IS Routing Protocol

 Today I am going to talk about Intermediate System-Intermediate System (IS-IS) routing protocol. Some of you already knew about the IS-IS routing protocol and some are new to this. Well IS-IS stands for Intermediate system to intermediate system. IS-IS is a dynamic routing protocol and is under link state type of dynamic routing protocol.

IS-IS protocol is extensively used as Interior Gateway Protocol (IGP) in Internet Service Provider (ISP) environment. The scope of this document is to provide information regarding IS-IS area types, configuration and troubleshooting. 

In Cisco world Integrated IS-IS is deployed, meaning IS-IS is routing Internet Protocol (IP) .In this document term IS-IS means ‘Integrated IS-IS’. 

The real power of IS-IS lies in its use of TLVs (Type-Length-Value) making IS-IS highly extensible protocol. As new features come in, they can be added to protocol using TLVs.

Fig 1.1- IS-IS routing Protocol Network Topology

MTU : If one IS-IS router receives an ISIS hello packet with higher MTU than it can support (on the interface) it discards the hello hence the adjacency doesn’t come up. In best practice MTU must be same on both the ends.

Circuit Type : This attribute is configured on interface and defines what type of hellos i.e. L1 or L2 are sent on a particular interface. A L1/L2 router can selectively send L1 only hellos on one interface and L2 only hellos on its other interface. 

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: IS-IS can 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 won’t exchange. So authentication if configured for IS-IS 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.

Network Type: There are only two network types in IS-IS. Broadcast and Point-to-Point. Broadcast is default network type. 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.

Hellos: Hello timers need not match for the adjacency to come up.

IS-IS protocol has two levels or hierarchy, Level 1 and Level 2. 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 Level 1 (L1) Router
An IS-IS Level 1 router has the link state information of its own area for all the intra-area topology. In order to route packets to other areas it uses the closest Level 2 capable (L1/L2) router. Level 1 Area behaves pretty much as OSPF totally stubby area. L1 only router send L1 Hellos.

IS-IS Level 1-2 (L1/L2) Router
An IS-IS L1/L2 router maintains two link state database information. One is for Level 1 and the other for Level 2.Hence two distinct Shortest Path First (SPF) calculations are run, one on Level 1 link state database and other on the Level 2 link state database. IS-IS Level 1-2 router behaves very close to OSPF Area Border Router (ABR). L1/L2 router sends both L1 and L2 hellos.

As default behaviour L1/L2 router will only allow one way passage of prefixes from L1 Area to L2 Area, but not in reverse. However if it is required to move prefixes from L2 Area to L1 Area then redistribute command under IS-IS configuration is required.

IS-IS Level 2 (L2) Router
An IS-IS Level 2 router has the link state information for the intra-area as well as inter-area routing. L2 router sends only L2 hellos. IS-IS Level 2 area can be compared with OSPF backbone area 0.

IS-IS Adjacency States
There are only three adjacency states in IS-IS.

Down: This is the initial state. Its means that no hellos have been received from the neighbor.

Initializing: This state means that the local router has successfully received hellos from the neighboring router, however it’s 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.