IS-IS Adjacency


IS-IS Protocol Adjacency

Routing protocols need to establish neighborship to operate. IS-IS Protocol needs also adjacency for this purpose. In this lesson, we will learn IS-IS Adjaceny mechanims and IS-IS Protocol neighborhip establishment.


There are three types adjacency in IS-IS Protocol.These are:

  • Level 1 routers form L1 adjacency with L1 and L1/2 Routers
  • Level 2 routers form L2 adjacency with L2 and L1/2 Routers
  • Level 1/2 routers form L1/2 adjacency with L1/2 Routers


There is no neighbourship between L1 and L2 routers.


In IS-IS Protocol, adjacency is established with 3-way handshake mechanism. There are three states in IS-IS adjacency. Let’s check the adjacency states of IS-IS Protocol.


In the first place the router adjacency is in Down State. One of the routers, here Router A sends a Hello PDU. When the router B receive this Hello including the MAC address of the sending node Router A, it goes to Initialize State and sends Hello to Router A too. Again, when Router A receives Hello including the MAC address of Router B, bidirectional communication established. And the name of this state is Up State.


IS-IS Adjacency States, IS-IS Neighbourship Establishment

IS-IS Protocol Adjacency States
As a summary:

  • No IS-IS neighbour (Down State)
  • RTRA Sends Hello (Initializing State)
  • RTRB records MAC address f RTRA and Sends Hello too. RTRA sees MAC address of RTRB. Bidirectional communication established (Up State)



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IS-IS Protocol Operation

Beginning with IS-IS adjacency establishment, to the Routing Table establishment, IS-IS Operation has some steps.
We can summarize IS-IS operation in 6 steps. These IS-IS operation steps are given below:

IS-IS Protocol Operation
IS-IS Protocol Operation

  1. IS-IS Routers are send Hello PDUs to discover the neighbours and establish the adjacency.
  2. IS-IS adjacency is established (Mainly authentication, IS-type, MTU must match)
  3. LSPs are build by routers about theirselves and the learned adjacent routers
  4. Routers send the LSPs to the adjacent routers.
  5. All routers build their LSDB according to these LSPs
  6. By SPF algorithm best paths are calculated and routing table is build.



DIS (Designated IS)

In ISIS network, LSDB databases must be synronized. To do this, full mesh or another solution is required. So, in IS-IS Protocol, one router is selected as DIS (Designated IS) and DIS is used to reduce the adjacency in areas. It is like DR in OSPF.


Router with the highest priority and then the highest MAC address is elected as DIS. The default DIS priority is 64. There is no BDR like OSPF. And like OSPF, if you want to prevent the router to be DIS, you can set the priority 0. If a new router added to the ISIS network, the election occurs again. This is not like this in OSPF.


DIS creates a Pseudo Node that is a virtual router. All connected devices in that LAN establish neighbourship with this Pseudo Node.


IS-IS Protocol DIS Election
IS-IS Protocol DIS Election

DIS is used on multipoint-to-multipoint topologies. It is not used in point-to-point topologies. Seperate DISs are selected for Level 1 and Level 2.


Lesson tags: routing, is-is
Back to: CCIE Enterprise Infrastructure > IS-IS

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