A brief flashback of IS-IS – Part II

Hello PDU’s of IS-IS:

IS-IS has three types of Hello PDUs:

i. ESH (ES Hello – sent by ES to an IS)

ii. ISH (IS Hello – sent by IS to an ES)

iii. IIH (IS-IS Hello – sent between two ISs)



Routing Levels

Level0: Used to locate end systems

Level1: Routing within an area

Level2: Backbone between areas

Level3: Inter-AS routing



NSAP Addressing

IS-IS uses the NSAP address for communication. Each router in IS-IS is identified with an ID called NET (Network Entity Title) same as Router ID which is NSAP address. Below is the NSAP addressing format:

Below are field descriptions:

AFI (First 8 bits): Any number (usually 49 indicating Private IS-IS)

Area (16 bits): Area

System ID (48 bits): MAC Address of any interface of this Router

NSEL (8 bits): Zero (NSEL = 00 means the device itself. The NSAP with a NSEL = 00 is known as a NET)


Cisco format for NSAP Addressing is: 

AFI.Area . System ID(MAC) . NSEL (always 00 on ISs)

(e.g. 49.0001.2222.2222.2222.00)



IS-IS Routing Process

IS-IS Routing Process is divided into four stages:

i. Update: LSPs are generated and flooded throughout the network whenever there is a change in network (adjacency changes, interface up/down or metric changed, route changes)

ii. Decision: Create SPT (Shortest Path Tree) & Routing table based on Dijkstra’s algo

iii. Forwarding: After the SPT has been built the forwarding database is created

iv. Receive: After receiving a frame, IS-IS checks it & if the frame is valid, the receive process passes user data and error reports to the forwarding process. Whereas routing information (Hellos, LSPs, and SNPs) are sent to the update process


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11 Comments on "A brief flashback of IS-IS – Part II"

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Luke Kai

Good. Concepts simplified!!


Most of new students have a short idea about IS-IS topic. It is a very good post for them. keep it up…..


I have a confusion on the definition of Packet Loss Rate.
Is it the number of packets lost per unit of time or number of packets lost per number of packets sent?

Waqas Karim

Both are valid. If unit is not mentioned then it will assume as:
(no. of packets lost) / (no. of packets sent)

Louis Elliott

Thanks for giving this knowledgeable information. You really finished my confusion about network interfaces and end points. Thanks again!….

Waqas Karim

You’re welcome.

Louis Elliott

What are the endpoints network interface called at Data Link layer?

David Toby

As I know in IS-IS the router knows the interface addresses of the other routers. So is it possible to connect two IPv6 areas over an IPv4 area?