Adjacencies in IS-IS

IS-IS Protocol has two types of nodes (since the roots of IS-IS are based on ISO Connectionless Network Protocol) i.e. IS & ES.

• ES (End System): A workstation or network host (limited routing capability)
• IS (Intermediate Systems): Network devices such as routers with full packet-forwarding capabilities. The word intermediate refers to the capabilities of routers as intermediate forwarding or relay devices. These are also referred to as gateway in some older networking literature of ISO

*If these terms are new to you, I would recommend to review the IS-IS terminology & basics at ATech before proceeding further at:

A brief flashback of IS-IS – Part I

IS-IS was designed to provide routing intelligence for intermediate systems. The subnetwork-dependent functions of the routing layer provided by IS-IS are responsible for discovering, establishing, and maintaining adjacencies between the routers in an IS-IS domain.

As previously stated in IS-IS initial articles at ATech, IS-IS works in conjunction with ES-IS and certain elements of the CLNP protocol to achieve this. No special configuration is required on Cisco routers to enable ES-IS. The ES-IS operation is enabled automatically when IS-IS is configured on routers, and it runs as a background process to support the operation of IS-IS. The subnetwork-dependent functions of IS-IS work with ES-IS to determine network layer addresses of all adjacent neighbors (both end systems and routers). IS-IS establishes two types of adjacencies:

1. ES-IS Adjacencies

In IP environments, ES-IS is not much significant. It only facilitates router-to-router adjacency formation. IP hosts do not participate in the ES-IS protocol and instead rely on ARP for Layer 3-to-Layer 2 address resolution in determining the Layer 2 addresses of LAN-connected hosts and the IP default gateway.

2. IS-IS Adjacencies

IS-IS adjacency is the real IS-IS routing adjacency as in other IGP’s. Successful formation of an IS-IS adjacency between two nodes allows the exchange of IS-IS routing information.

These are the rules to form successful adjacency:

• L1 routers form L1 adjacencies with L1 and L1-L2 routers in their area
• L2 routers form L2 adjacencies with L2 and L1-L2 routers in their area or another area
• L1/L2 routers form L1 and L2 adjacencies with each other in their area or another area
• An L1 router does not form an adjacency with an L2 router, regardless of area
• The system ID must be unique to each router
• Hello intervals and hold times do not have to match

IS-IS adjacencies on point-to-point links are initialized by receipt of ISHs through the ES-IS protocol. This is followed by the exchange of point-to-point IIHs. The type of adjacency formed will depend on the parameters exchanged in the IIHs.

Below is a summary mentioning clearly who can form an adjacency & who cannot: