Concept and structure of IS-IS routing protocol (1)

IS-IS routing protocol we can learn from the knowledge of OSPF when learning, in some ways these two types of protocols are very similar. Now let's take a closer look at this aspect. First, we all know that in the ISO specification, a router IS an IS (intermediate system), and a protocol that provides communication between IS and IS (router and router) IS a routing protocol, that IS, the IS-IS routing protocol. The IS protocol IS very similar to the OSPF protocol and IS a link-State routing selection protocol. In fact, it appears earlier than the OSPF prototype, ospf is used in the route selection for Internet and TCP/IP network IP communication. IS-IS can be used in IP communication, it can also be used in OSI communication and can provide dynamic routing for Information Transmission groups between two routers in the same domain ｡

The so-called integrated IS-IS introduction the intra-domain routing information exchange protocol between the Intermediate System and the Intermediate System IS-IS (Intermediate System-to-Intermediate System intra-domain routing information exchange protocol) it was originally a dynamic routing Protocol designed by ISO for its connectionless Network Protocol CLNP (Connection-Less Network Protocol, to provide IP Routing Support, IETF expands and modifies the IS-IS in RFC1195 so that it can be applied to both TCP/IP and OSI environments, IS called Integrated IS-IS (Integrated IS-IS or Dual IS-IS )｡

The IS-IS routing protocol IS a link status protocol. It uses the Shortest Path First (Shortest Path First) algorithm, there are many similarities with the OSPF Protocol. The IS-IS routing Protocol belongs to the Internal Gateway Protocol IGP (Interior Gateway Protocol), used inside the autonomous system ｡

IS-IS Concept

(1) IS-IS routing protocol terminology

IS (Intermediate System, Intermediate System). equivalent to a router in a TCP/IP, it IS the basic unit for generating routing information in the IS-IS protocol and transmitting routing information. It has the same meaning as the router in the following article ｡

ES (End System). It IS equivalent to a host System in TCP/IP. ES does not participate in the IS-IS routing protocol, ISO has a dedicated ES-IS protocol to define the communication between the terminal system and the intermediate system ｡

RD (Routing Domain, Routing Domain). In a Routing Domain, a group of IS exchange Routing information through the same Routing protocol ｡

Area. The unit of the route field ｡

LSDB (Link State DataBase, Link State DataBase). All network connection States constitute a Link State DataBase, and each IS has at least one LSDB. IS uses the SPF algorithm, use LSDB to generate your own route ｡

LSPDU (Link State Protocol Data Unit). In IS-IS, each IS generates an LSP, this LSP contains all the link status information of this IS. Each IS collects all LSPs in the region to generate their own LSDB ｡

Network Protocol Data Unit (NPDU) IS a Network layer Protocol packet in ISO, which IS equivalent to an IP packet in TCP/IP. DIS (Designated IS) that is, the specified intermediate system of the broadcast network ｡

Network Service Access Point (NSAP) is the Network layer address in ISO. It is used to identify an abstract Network layer Access Service Point and describe the Network address structure of the ISO model ｡

(2) link types applicable to IS-IS Routing Protocol

IS-IS can run on Point-to-Point Links, such as PPP and HDLC, or Broadcast Links, such as Ethernet and Token-Ring, for NBMA (Non-Broadcast Multi-Access) networks, such as ATM, You need to configure sub-interfaces for them, configure the sub-interface type as P2P or broadcast network. IS-IS cannot run on a Point to Multi Point Links ｡

IS-IS routing protocol structure

To support large-scale routing networks, IS-IS uses two layers in the routing domain. A large routing domain IS divided into one or more regions. routes in the region are managed through the Level-1 router, routes between regions are managed through Level-2 routers ｡

(1) Level-1 Router