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: … Continue reading A brief flashback of IS-IS – Part II