5.3.1 Internet Routing Protocols (1)
We know that the Internet is widely used in real life. We can chat online, query information, shop, etc. It brings great convenience to our real life. We already know that the Internet is a network connected by routers. Combined, for users, the messages sent between each other are actually forwarded by the routing table according to the routing control table. And such an important routing control table is like the picture below
Something as important as the road signs in the network, which is established by the routing protocol we learned today.
1. Routing protocol
**The routing protocol is a pre-agreed regulation and standard that guides the sending of IP datagrams. **Its role is to describe the network topology, coordinate routers for routing selection and datagram forwarding, and we will learn about routing protocols from four aspects.
- Classification of Autonomous Systems and Routing Protocols
- Interior Gateway Protocol RIP, which is a Routing Information Protocol based on the Distance Line Algorithm
- Interior Gateway Protocol OSPF, which is based on Open Shortest Path First
- Exterior Gateway Protocol BGP, Border Gateway Protocol
2. Classification of autonomous systems and routing protocols
Considering the adaptability of routing algorithms to network changes, routing algorithms can be divided into two types, namely static routing (non-adaptive routing algorithm) and dynamic routing (adaptive routing algorithm) .
In the case of static routing, the routing table of the router is manually configured by the administrator. It is generally used for relatively simple interconnected networks. The biggest advantage of static routing is simplicity and low overhead. The disadvantage is that once the topology of the network changes Or if a link fails, the network administrator must manually modify the routing table; otherwise, some packets cannot be routed and forwarded, that is, they cannot change with the network.
Dynamic routing is to enable routers to dynamically adapt to changes in link status and network topology. Current routers support dynamic routing, and dynamic routing is realized through the routing protocol we have learned. Its advantage is that it can better The disadvantage of adapting to changes in network status is that it is more complicated to implement and has a higher overhead.
3. Autonomous system
Before learning the routing protocol, let's learn a basic concept----Autonomous System (Autonomous System) is also called autonomous domain, referred to as AS. The definition given in the RFC4271 document is that an autonomous system is in a management organization Routers and network groups under control .
The figure is an example of an autonomous system. Autonomous system 1 on the left has a router directly connected to two LANs, and is also connected to autonomous system 2 on the right. Autonomous system 2 on the right is connected by two routers. Consists of multiple local area networks.
Fourth, the classification of routing protocols
From this example, we know that all routers in an autonomous system must be connected to each other. According to whether the routing protocol is used within the autonomous system or between different autonomous systems, we divide the routing protocol into two types, namely the interior gateway protocol. (IGP) and Exterior Gateway Protocol (EGP) , the protocol used when exchanging routing information between routers within the autonomous system is called Interior Gateway Protocol (IGP), and vice versa the protocol used between different autonomous systems is called Become an Exterior Gateway Protocol. as shown in the picture
There are three autonomous systems in the figure. In autonomous system A, each router obtains information through an interior gateway protocol such as routing information protocol RIP, and the same is true for autonomous system B and autonomous system C. The external gateway protocol is used between different autonomous systems, that is to say, the border routers of each autonomous system obtain the routing information to other autonomous systems through the external gateway. If they are located between hosts H1 and H2 of different systems For communication, the routers forwarding data along the way must first obtain a route to the border router of the autonomous system through the interior gateway protocol, then obtain the route to the border router of the autonomous system through the exterior gateway protocol, and finally pass The interior gateway protocol obtains the route to the destination host to complete the communication process.
Later we will learn the two representatives of the interior gateway protocol, the RIP protocol and the OSPF protocol, and the BGP protocol, the representative of the exterior gateway protocol.