1 Overview
Routing is divided into static routing and dynamic routing, and its corresponding routing tables are called static routing tables and dynamic routing tables. The static routing table is preset by the network administrator according to the network configuration during system installation. After the network structure changes, the network administrator manually modifies the routing table. Dynamic routing changes with the changes in network operation. The router automatically calculates the best path for data transmission based on the functions provided by the routing protocol, thereby obtaining a dynamic routing table.
According to routing algorithms, dynamic routing protocols can be divided into Distance Vector RouTIng Protocol and Link State RouTIng Protocol. The distance vector routing protocol is based on Bellman-Ford algorithm, mainly RIP, IGRP (IGRP is Cisco's private protocol); the link state routing protocol is based on the well-known Dijkstra algorithm in graph theory, namely the shortest priority path (Shortest Path First, SPF) ) Algorithms such as OSPF. In the distance vector routing protocol, the router passes part or all of the routing table to the neighboring router; in the link state routing protocol, the router passes the link state information to all routers in the same area. According to the position of the router in the autonomous system (AS), the routing protocols can be divided into Interior Gateway Protocol (IGP) and External Gateway Protocol (EGP, also known as Inter-Domain Routing Protocol). There are two inter-domain routing protocols: External Gateway Protocol (EGP) and Border Gateway Protocol (BGP). EGP is designed for a simple tree topology. It has obvious shortcomings when handling routing loops and setting routing strategies. It has been replaced by BGP.
EIGRP is Cisco's private protocol and is a hybrid protocol. It not only has the characteristics of distance vector routing protocol, but also inherits the advantages of link state routing protocol. Each routing protocol has its own characteristics and is suitable for different types of networks. Each is explained below.
2 Static routing
The static routing table is established by the network administrator before starting to select the route, and can only be changed by the network administrator, so it is only suitable for the environment where the network transmission status is relatively simple. Static routing has the following characteristics:
Static routing does not require routing exchange, so it saves network bandwidth, CPU utilization, and router memory.
Static routing has higher security. In a network using static routing, all routers to be connected to the network need to set their corresponding routes on the adjacent routers. Therefore, the security of the network is improved to some extent.
In some cases, static routing must be used, such as DDR, a network environment using NAT technology.
Static routing has the following disadvantages:
The administrator must truly understand the topology of the network and configure the routing correctly.
The network has poor scalability. If you want to add a network to the network, the administrator must add a route to all routers.
The configuration is cumbersome, especially when it needs to communicate across several routers, the routing configuration is more complicated.
3 Dynamic routing
Dynamic routing protocols are divided into distance vector routing protocols and link state routing protocols. Both protocols have their own characteristics and are described below.
1. Distance Vector (DV) Protocol
Distance vector refers to a protocol that uses hops or vectors to determine the distance from one device to another. The rate of the link per hop is not considered.
The distance vector routing protocol does not use the normal neighbor relationship, and uses two methods to learn the topology change and the routing timeout:
When the router cannot directly receive routing updates from the connected router;
When a router receives an update from a neighbor, it informs it that the topology has changed somewhere in the network.
In small networks (less than 100 routers, or requiring fewer routing updates and computing environments), the distance vector routing protocol works quite well. When the small network is extended to a large network, the algorithm calculates the convergence speed of the new route extremely slowly, and in the process of its calculation, the network is in a transitional state, and loops are likely to occur and cause temporary congestion. Furthermore, when the underlying link technology of the network is diverse and the bandwidth is different, the distance vector algorithm ignores this.
This feature of the distance vector routing protocol not only causes a delay in network convergence, but also consumes bandwidth. As the routing table increases, more CPU resources are consumed and memory is consumed.
2. Link State (LS) routing protocol
The link state routing protocol has no limit on the number of hops, and uses the "graphic theory" algorithm or the shortest path first algorithm.
The link state routing protocol has a shorter convergence time and supports VLSM (Variable Length Subnet Mask) and CIDR.
Link-state routing protocols maintain normal neighbor relationships between directly connected routes. This allows routing to converge faster. The link state routing protocol creates a peer-to-peer relationship by exchanging Hello packets (also called link state information) during a session. This relationship speeds up route convergence.
Unlike the distance vector routing protocol, the entire routing table is sent when updating. The link state routing protocol only broadcasts updated or changed network topology, which makes the updated information smaller, saving bandwidth and CPU utilization. In addition, if the network does not change, the update package is only sent within a specific time (usually 30min to 2h).
3. Comparison of link state routing protocol and distance vector routing protocol
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