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