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Classification of dynamic routing protocols

Classification by work area:

• 「IGP (Interior Gateway Protocols): RIP OSPF IS-IS」
• 「EGP (Exterior Gateway Protocols): BGP」

Classification by working mechanism and algorithm:

• 「Distance Vector Routing Protocol: RIP」
• 「Link State Routing Protocol: OSPF IS-IS」

• BGP uses an algorithm modified from the distance vector algorithm, which is called the path vector algorithm. Therefore, in some cases, BGP is also called the path vector routing protocol.
• IGP (Interior Gateway Protocol) exchanges routing information within the same autonomous system. The main purpose of IGP is to discover and calculate routing information within the autonomous domain.
• EGP (External Gateway Protocol) is mainly used for interconnection between ASs (Autonomous Systems).

Distance Vector Routing Protocol

Routers running distance vector routing protocols "periodically" flood their routing tables. Through routing interactions, each router learns routes from adjacent routers, loads them into its own routing table, and then announces them to other adjacent routers. For all routers in the network, the routers do not know the network topology, but simply know where to go to a certain destination network segment and how much the cost is.

Link State Routing Protocol

LSA Flooding

• Link state routing protocols advertise "link status" rather than routing information.
• Routers running link-state routing protocols first establish neighbor relationships and then begin to exchange LSAs (LinkState Advertisements) with each other.
• Link state advertisement can be simply understood as each router generating a notification describing the status of its directly connected interface (including the interface cost, the relationship with the neighboring router, etc.).

LSDB maintenance

Each router generates LSAs, and the router stores the received LSAs in its own LSDB (Link State DataBase). The router understands the entire network topology by parsing the LSAs stored in the LSDB.

SPF calculation

Each router uses the SPF (Shortest Path First) algorithm to calculate based on the LSDB. Each router calculates a "tree" with itself as the root, no loops, and the shortest path. With this "tree", the router already knows the preferred path to reach all network segments.

SPF is a core algorithm of the OSPF routing protocol, which is used to make routing optimization decisions in a complex network.

Routing table generation

The router loads the calculated preferred path into its own routing table.

Summary of Link State Routing Protocols

Link state routing protocols have four steps:

• The first step is to establish neighbor relationships between adjacent routers.
• The second step is to exchange link state information and synchronize LSDB between neighbors.
• The third step is to calculate the optimal path
• The fourth step is to generate routing table entries based on the shortest path tree and load them into the routing table