Analysis of the function and architecture of routers

One of the functions of a router is to connect to different networks, and the other is to select the route of information transmission. Choose smooth shortcut, can greatly improve communication speed, reduce network system communication load, save network system resources, improve network system unblocked rate, so that the network system to play a greater benefit.

From the point of view of filtering network traffic, the function of router is very similar to that of switch and Network Bridge. However, unlike the switches that work on the network physical layer and physically divide the network segment, routers use specialized software protocols to logically divide the entire network. For example, a router that supports IP protocols can divide the network into multiple subnet segments, and only network traffic that points to a particular IP address can pass through the router. For each received packet, the router recalculates its checksum and writes the new physical address. Therefore, the speed with which data is forwarded and filtered by routers is often slower than the switch that only looks at the physical address of the packet. However, for those networks with complex structures, using routers can improve the overall efficiency of the network. Another obvious advantage of routers is that they can automatically filter webcasts. Overall, the entire installation process of adding routers to a network is much more complex than a Plug and play switch.

Generally speaking, the interconnection of heterogeneous networks and multiple subnets should be done by routers.

The main task of the router is to find an optimal transmission path for each data frame passing through the router and to effectively transfer the data to the destination site. This shows that the choice of the best path strategy is the routing algorithm is the key to the router. In order to complete; this work, in the router holds the various transmission path the correlation data--The Path table (Routing table), uses for the route choice; The path table holds the logo information of the subnet, the number of routers on the network, and the name of the next router. The path table can be fixed by the system administrator, or can be dynamically modified by the system, can be automatically adjusted by the router, can also be controlled by the host.

1. Static Path table

By the system administrator set up a fixed path table is called a static path table, is generally installed in the system according to the configuration of the network pre-set, it will not change with the future network structure changes.

2. Dynamic Path table

The Dynamic path table is a path table that routers automatically adjust according to the network system's operation. Routers provide the functions of the routing protocol (Routing Protocol) to automatically learn and memorize network operations and automatically calculate the best path for data transfer when needed.

The architecture of the router

From the view of architecture, routers can be divided into first generation single-bus single CPU router, second generation single bus CPU router, third generation single bus symmetric multi CPU structure router, fourth generation multi-bus multi-CPU router, fifth generation shared memory structure router, The sixth generation of cross switch architecture routers and routers based on cluster system are many kinds.

　　 the composition of the router 　

Routers have four elements: input ports, output ports, switching switches, and routing processors.

The input port is the physical link and the import of the input package. Ports are usually provided by a wire card, and a single line card typically supports 4, 8, or 16 ports, and one input port has many features. The first feature is the encapsulation and encapsulation of the data link layer. The second function is to find the destination address of the input package in the forwarding to determine the destination port (known as routing lookup), which can be implemented using general hardware, or by embedding a microprocessor on each line card. Third, in order to provide QoS (quality of service), the port is divided into several predefined service levels for the packets received. Finally, ports may need to run data link-level protocols such as SLIP (Serial Line Internetwork Protocol) and PPP (Point-to-Point Protocol) or network-level protocols such as PPTP (Point-to-Point Tunneling Protocol). Once the routing lookup is complete, the packet must be sent to its output port with a switching switch. If the router is an input-side plus queue, there are several inputs that share the same switch. The final function of this input port is to participate in the arbitration agreement for public resources such as switching switches.

Switching switches can be implemented using a variety of different techniques. The switch technology that has been used up to now is bus, crossover switch and shared memory. The simplest switch uses a single bus to connect all the input and output ports, and the disadvantage of the bus switch is that its exchange capacity is limited by the capacity of the bus and the additional overhead of quorum for the shared bus. A crossover switch provides multiple data paths through a switch, and a crossover switch with a NXN intersection can be considered to have a 2N bus. If a crossover is closed, data on the input bus is available on the output bus, otherwise it is unavailable. The closure and opening of intersections are controlled by the scheduler, so the scheduler restricts the speed of switching. In a shared memory router, incoming packets are stored in a shared memory, swapping only the packets ' pointers, which increases the exchange capacity, but the speed of the switch is limited by the memory's access speed. Although memory capacity can be doubled every 18 months, memory access time is reduced by only 5% a year, which is an inherent limitation of shared memory switching switches.

The output port is stored on the packet before it is sent to the output link, and a complex scheduling algorithm can be implemented to support the priority requirements. As with the input port, the output port also supports the package and encapsulation of the data link layer, as well as many of the more advanced protocols.

The routing processor calculates the routing protocol and runs the software that configures and manages the router. At the same time, it also handles those packets whose destination address is not in the online card forwarding publication.