Routers provide a mechanism for interconnecting heterogeneous networks, enabling data packets from one network to be sent to another network. Routing is the path information that guides the sending of IP packets. The routing protocol is the rules and standards agreed in advance in the process of routing IP data packets.
Routing protocols support routable protocols by sharing routing information between routers. Routing information is passed between adjacent routers to ensure that all routers know the path to other routers. In short, the routing protocol creates a routing table that describes the network topology; the routing protocol works with routers to perform routing and packet forwarding functions.
Classification of common routing protocols
Gateway-Gateway Protocol (GGP)
In order to route packets correctly and efficiently, the core gateway needs to know what is happening in other parts of the Internet, including routing information and subnet characteristics.
When a gateway handles heavy loads and is particularly slow, and this gateway is the only way to access the subnet, this type of information is usually used, and other gateways in the network can tailor traffic to reduce the gateway's load.
GGP is mainly used to exchange routing information, do not confuse routing information (including address, topology and routing delay details) and the algorithm to make routing decisions. The routing algorithm is usually fixed within the gateway and is not changed by GGP. The core gateways communicate by sending GGP information and waiting for a response, and then update the routing table if a response with specific information is received.
Note that GGP's latest improvement, SPREAD, has been used on the Internet, but it is not as popular as GGP. GGP is called a vector-distance protocol. To work effectively, the gateway must contain complete information about all gateways on the Internet. Otherwise, it is impossible to calculate the effective route to a destination. For this reason, all core gateways maintain a list of all core gateways on the Internet. This is a fairly small table that the gateway can easily handle.
Exterior Gateway Protocol (EGP)
The external gateway protocol is used to transfer information between non-core adjacent gateways. Non-core gateways contain routing information of all directly adjacent gateways on the Internet and information about connected machines, but they do not contain information of other gateways on the Internet. For most EGPs, only the information of the local area network or wide area network that maintains their services is restricted. This prevents excessive routing information from being transmitted between LANs or WANs. EGP forces exchange of routing information between non-core gateways.
Since the core gateway uses GGP and the non-core gateway uses EGP, and both are applied on the Internet, there must be some method to enable the two to communicate with each other. The Internet enables any autonomous (non-core) gateway to send "reachable" information to other systems, which must be sent to at least one core gateway. If there is a larger autonomous network, it is often considered that there is a gateway to handle these reachable information.
Like GGP, EGP uses a query process to make the gateway aware of its neighbors and continuously exchange routing and status information with its neighbors. EGP is a state-driven protocol, meaning that it relies on a state table that reflects the condition of the gateway and a set of operations that must be performed when the state table entry changes.
Internal Gateway Protocol (IGP)
There are several internal gateway protocols available, the most popular are RIP and HELLO, and the other protocol is called Open Shortest Path First (OSPF). None of these protocols are dominant, but RIP is probably the most common IGP protocol . The selection of a specific IGP is based on the network architecture.
Both RIP and HELLO protocols calculate the distance to the destination, and their messages include the machine identification and the distance to the machine. Generally speaking, since their routing table contains many items, the message is relatively long. RIP and HELLO have always maintained the connectivity between adjacent gateways to ensure that the machine is active.
The routing information protocol uses broadcast technology. This means that the gateway broadcasts the routing table to other gateways at regular intervals. This is also a problem of RIP, because it will increase network traffic and reduce network performance.
The difference between HELLO and RIP is that HELLO uses time instead of distance as a routing factor. This requires the gateway to have reasonable and accurate time information for each route. For this reason, the HELLO protocol relies on clock synchronization messages.
The Open Shortest Path First protocol is a protocol developed by the Internet Engineering Task Force, and it is hoped that it will become the dominant IGP. Using "shortest path" to describe the routing process of the protocol is inaccurate. A better name is "optimal path", which must consider many factors to determine the best route to the destination.
The role of routing protocols
The routing protocol mainly runs on the router. The routing protocol is used to determine the arrival path. It includes RIP, IGRP, EIGRP, and OSPF. It acts as a map navigation and is responsible for finding the way. It works at the network layer.
The routing protocol is mainly a protocol running on the router, which is mainly used for path selection.
The routing protocol is one of the important members of the TCP / IP protocol family. The quality of the routing process will affect the efficiency of the entire Internet network. According to different application areas, routing protocols can be divided into two categories: in an AS (Autonomous System, autonomous system, refers to an interconnected network, that is, the entire Internet is divided into many smaller network units, these small networks are entitled to autonomous To decide which routing protocol should be used in this system) The routing protocol is called interior gateway protocol, and the routing protocol between ASs is called exterior gateway protocol. The gateway here is the old name of the router. There are several internal gateway routing protocols currently in use: RIP-1, RIP-2, IGRP, EIGRP, IS-IS, and OSPF. The first four routing protocols use distance vector algorithm, IS-IS and OSPF use link state algorithm. For small networks, the routing protocol based on the distance vector algorithm is easy to configure and manage, and is widely used. However, in the face of large networks, not only its inherent loop problems become more difficult to solve, but the bandwidth occupied also increases rapidly , So that the network cannot bear it. Therefore, for large networks, IS-IS and OSPF using link state algorithms are more effective and are widely used. The difference in quality and performance between IS-IS and OSPF is not large, but OSPF is more suitable for IP and more dynamic than IS-IS. The IETF has always been committed to improving OSPF, and its revision pace is much faster than IS-IS. This makes OSPF is becoming a widely used routing protocol. Now, whether it is a traditional router design or the MPLS (Multi-protocol Label Switching) that is about to become a standard, OSPF is regarded as an essential routing protocol.
The external gateway protocol originally adopted EGP. EGP is designed for a simple tree topology. As more and more users and networks join the Internet, it brings many limitations to EGP. In order to get rid of the limitations of EGP, the IETF Border Gateway Protocol Working Group has developed a standard border gateway protocol-BGP.
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