5.3.4 Internet routing protocol (4) BGP protocol

5.3.4 Internet routing protocol (4) BGP protocol

The RIP protocol we studied ( 5.3.2 Internet routing protocol (2) RIP protocol based on distance vector algorithm ) and OSPF protocol ( 5.3.3 Internet routing protocol (3) OSPF protocol ) are both interior gateway protocols. It can be used in an autonomous system, how to realize the transmission of information between different autonomous systems, which requires external gateway protocols such as BGP as a bridge for information transmission between two autonomous systems, and the BGP protocol is also called Border Gateway Protocol , which is a routing protocol running on top of TCP, BGP is the only protocol used to deal with networks as large as the Internet, and it is also the only protocol that can properly handle multi-path connections between unrelated routing domains, and RIP protocol The difference between OSPF and BGP is that BGP adopts a path vector routing algorithm to find a relatively good route to the destination network or autonomous system, instead of finding a route with the least number or total number of routers like RIP and OSPF. The best route with the least cost.

1. The principle of communication between different autonomous systems using the BGP protocol

as shown in the picture

There are three different autonomous systems in the figure, namely AS100, AS200, and AS300. The interior gateway protocol IGP ( 5.3.1 Internet routing protocol (1) ) is used inside the autonomous system, and the external gateway protocol is used between autonomous systems. The gateway protocol EGP ( 5.3.1 Internet routing protocol (1) ), when the three autonomous systems need to communicate with each other, each must choose a border router as the Speaker of BGP, also known as the spokesperson of BGP, In this way, the purpose of mutual communication between different autonomous systems can be achieved by using the BGP speaker.

Second, the characteristics of the routing protocol

1. The RIP protocol , which is a distributed routing protocol based on the distance vector algorithm, is only suitable for small autonomous systems, the maximum number of hops is 15, and multiple routes cannot be used at the same time, and they will exchange their respective routes with neighboring routers every 30 seconds. entire routing table. Here we should note that this protocol works at the application layer, and the UDP protocol is selected at the transport layer.
2. OSPF protocol , which is a routing protocol based on link state, uses Dijkstra's shortest path algorithm and updates link state information through flooding, and all routers maintain a link state database. This protocol works at the network layer . It is the same as the ICMP protocol we introduced earlier ( 5.2.7 Internet Control Message Protocol ICMP ), OSPF message is encapsulated in the data part of IP datagram . That is, it directly uses IP datagrams to transmit information.
3. The BGP protocol saves the path through the path vector algorithm. This protocol is mainly used to exchange information between different autonomous systems. It works at the application layer like RIP, but its transport layer uses the TCP protocol.