How it is composed of router? This question first about the architecture from the router.
Router Architecture
From an architectural point of view, the router can be divided into the first generation of single-bus single-CPU structure of the router, the second generation single-bus structure of the main router from the CPU, the third generation of a single symmetric multi-CPU bus router; fourth generation routers, multi-bus multi-CPU , the fifth generation routers shared memory structure, the sixth generation crossbar switch architecture, cluster-based router and the router and other types.
The composition of the router
Router has four elements: input ports, output ports, switching and routing switch processor.
Input port is the physical link and enter the package's entrance. Line card port is usually provided general support for a line card port 4,8 or 16, an input port has many features. The first feature is the data link layer of packing and unpacking. The second function is to find the input packet forwarding destination address to determine the destination port (called routing lookup), route lookup can be used to achieve the general hardware, or through the line in each block of a microprocessor embedded in the card to complete . Third, in order to provide QoS (quality of service), the port should receive the package into several predefined level of service. Fourth, the port may need to run such as SLIP (Serial Line Internet Protocol) and PPP (Point to Point Protocol) link-level data such agreement or as PPTP (Point to Point Tunneling Protocol) network-level protocol such. Once the routing lookup to complete, the package must be sent to the switch output port. If the router is to increase the input queue, there are several inputs share the same exchange switch. This input function is to attend the last public resources (such as the exchange switch) the arbitration agreement.
Exchange switch can use a variety of different technologies. So far the most used technology is the bus exchange switches, cross switches and shared memory. The simplest use of a bus switch to connect all input and output ports, the bus switch drawback is its exchange capacity is limited by the capacity of the bus and brought to a shared bus arbitration overhead. Crossbar switch through the switch to provide multiple data paths, one with N × N crosspoint switch cross-section can be considered to have 2N bus. If a cross is closed, the data input bus is available in the output bus, otherwise unavailable. Intersection of closed and open to control by the scheduler, thus, the scheduler limits the switching speed of the exchange. In the shared memory router, incoming packets are stored in shared memory in the exchange package is only a pointer, which increased the exchange capacity, however, switching speed is limited by memory access speed . Although the memory capacity to double every 18 months, but the memory access time of only 5% reduction each year, this is a shared memory limitations inherent in the switch.
Output port in the packet is sent to the output link on the package before storage, you can implement complex scheduling algorithm to support the priority requirements. As with the input port, output port also to be able to support the data link layer packing and unpacking, and many more advanced protocols.
Routing processor computing forwarding of routing protocols, and run on the router configuration and management software. It also does not deal with the line card destination address of the packet forwarding.