ISP domains
ISP domains

In this blog, we are going to discuss the IS-IS protocol which is widely used in large ISP domains. You can also refer to the LISP understanding that is used in SD-Access as a control plane.  The IS-IS protocol stands for “Intermediate System to Intermediate System”.

The following section will be covered in this blog:

  • IS-IS Basic Introduction & Functionalities
  • OSI & CLNP Address
  • IS-IS Level
  • IS-IS Router Type

The following are the basic functionalities of IS-IS protocol:

  1. It is an open standard protocol.
  2. It is an IGP Protocol.
  3. It is a link-state routing protocol.
  4. It is a layer 2 Protocol and uses Multicast MAC-Address for sending its messages.
  5. It uses the SPF algorithm for best path selection.
  6. It is a classless routing protocol.
  7. It supports subnetting and VLSM.
  8. The AD value of IS-IS is 115.
  9. The default Hello-Timer is 10sec and Dead-Timer is 30sec.
  10. It maintains a Neighbour table, Topology table and Routing table.
  11. By default, the cost value for all link types is 10. The IS-IS metric is not based on Link-Bandwidth, but we can manipulate the link cost ranging between <063>.

IS-IS History: The IS-IS protocol was initially developed for OSI address/protocol only. But later, it got modified and started supporting TCP/IP addresses as well.

Although, the OSI address is not being used in the present day. But still, we are required to OSI address if we run IS-IS on a router.

OSI Address: The OSI address is also known as the “CLNP” (Connection Less Network Protocol) address. Sometimes also called “Net-Address”.

CLNP/Net Address:  It can be 8-20 bytes in length. In Cisco Router, we can see 3 fields while configuring the CLNP address. The combination of these 3 fields is used to form a CLNP address.

The following are the 3 fields in CLNP/Net Address:

  1. AFI: Always set to 49 and for private use only.

Area-ID: It is at least 1 byte in length.

  • System-ID: It is just like a Router-ID used in OSPF. We can use the router MAC-Address as a system-id. It is 6 bytes in length.
  • NSEL (NSAP Selector): Always set to “00” while configuring the CLNP address on a router.
ISP domains

Note: We can assign CLNP address on a router, but not per interface based.

IS-IS Level:  The IS-SI has two Levels:

  1. Level-1: It is also known as Regular Area.
  2. Level-2: It is also known as Backbone Area. There is no concept of Area 0 in the IS-IS Backbone Area. It can be any Area number (i.e., 1,2,3,4 etc).

IS-IS Router Type: There are 3 types of routers in IS-SI as mentioned below:

  1. Level-1 Router: It is used for Internal/Intra communication. It acts as an Internal Router of OSPF.
  2. Level-2 Router: This router type is used as a backbone router to exchange routes between two areas.
  3. Level 1-2 Router: This type of router is used to communicate between two different areas just like the ABR router of OSPF.
ISP domains

This article explains the IS-IS protocol. We hope that while going through this article, you will be able to understand the basic concept of IS-IS protocol. This IS-IS protocol plays a vital role in SD-Access (SDA) for Underlay connectivity inside the SDA fabric. In continuation of this blog, we will discuss the IS-IS configuration on a router in the next blog for IS-IS Configuration Steps. You can also refer to another IS-IS blog.

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Sani Singh

Consultant – Enterprise Networking

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