A generic router has four components:
- Input ports: These are the point of attachment for a physical link and are the point of entry for incoming packets. Ports are instantiated on line cards, which typically support 4, 8, or 16 ports.
- Output ports: stores packets and schedules them for service on an output link.
- A switching fabric: It interconnects input ports with output ports.
- A routing processor: It participates in routing protocols and creates a forwarding table which is used in packet forwarding.
Types of Routers
Routers offer connectivity inside enterprises, between enterprises and the Internet, and inside Internet Service Providers (ISP). The largest routers, for example, the Cisco CRS-1 or Juniper T1600 interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks whereas the smallest routers provide connectivity for small and home offices. An edge router is a router which interfaces with an asynchronous transfer mode (ATM) network. A brouter is a network bridge combined with a router.
Also, depending on the relative speed of the input ports and the switching fabric a router can be classified as an:
- Output-queued: If the switching fabric has a bandwidth greater than the sum of the bandwidths of the input ports, then packets are queued only at the outputs, and the router is called an output-queued router.
- Input queued: Otherwise if the packets are queued up at the inputs, then the router is called an input-queued router.
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