TLV (Type-Length-Value or Tag-Length-Value) is an encoding scheme used for optional information element in a certain protocol. The type and length are fixed in size (typically 1-4 bytes), and the value field is of variable size. Type: It is simply the data being processed, a binary code, often simply alphanumeric, which indicates the kind of field that this part of the message represents. Length: It specifies the length/size of the value field, typically in bytes Value: Variable-sized series of bytes which contains data for this part of the message. It is the third field and contains a “length” amount of data representing the value for Read More …
Category: IS-IS
An OSI based IGP Routing Protocol
IS-IS Levels & Relationships
IS-IS creates flooding boundaries logical by breaking the network into routing domain levels i.e. A level2 routing domain and multiple level1 routing domains. The single Level 2 routing domain could be analogous to OSPF Area0 and multiple Level 1 routing domains are analogous to other OSPF Areas. Basically, IS-IS has two layer hierarchy: Level-2 (the backbone) Level-1 (the areas) This two layer hierarchy is achieved by dividing the routers (IS’s) into three types called L-1, L-2, L-1-2 IS’s. Level1 Router is analogous to OSPF Internal non-backbone router (Totally Stubby). It contains a level1 LSDB only and is responsible for only routing Read More …
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: Read More …
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 Read More …
Yet another difference b/w IS-IS and OSPF
Neighbour is someone who lives nearby, normally in a house or apartment that is next door or, in the case of houses, across the street. Some neighbors are adjacent while some are not. People form friendships with their neighbors, and help them by sharing their information & tasks. Similarly, in Network & routing world there is a difference in neighborship & adjacency. OSPF neighbourship is straightforward while IS-IS needs a bit deeper thought to get it fully. Lets take a closer look at both. For OSPF, in order to form OSPF adjacencies, the following must match on the neighboring routers: Read More …