A router goes one step further than a switch. It is a Layer 3 device that has much more intelligence than a hub or switch. By using logical Layer 3 addresses, routers allow devices on different LANs to communicate with each other and with distant devices—for example, those connected through the Internet or through a WAN. Examples of logical Layer 3 addresses include TCP/IP's IP addresses and Novell's IPX addresses.
A device connected to a router does not receive any of the information meant just for devices on other ports, or broadcasts (destined for all networks) from devices on other ports.
The router reads the source and destination logical addresses in the packets and therefore keeps track of who is where, and who is talking to whom, and sends data only where it needs to go. It supports communication between LANs, but it blocks broadcasts (destined for all networks).
All devices connected to one router port are in the same collision domain, but devices connected to different ports are in different collision domains.
All the devices connected to one router port are in the same broadcast domain, but devices connected to different ports are in different broadcast domains. Routers block broadcasts (destined for all networks) and multicasts by default; routers forward only unicast packets (destined for a specific device) and packets of a special type called directed broadcasts.
NOTE IP multicast technology, which enables multicast packets to be sent throughout a network, is described in Chapter 4, "Designing Basic Campus and Data Center Networks."
NOTE An IP-directed broadcast is an IP packet that is destined for all devices on an IP subnet. IP subnets are described in the "Addressing" section later in this chapter.
The fact that a router does not forward broadcasts (destined for all networks) is a significant difference between a router and a switch, and it helps control the amount of traffic on the network. For example, many protocols, such as IP, might use broadcasts for routing protocol advertisements, discovering servers, and so on. These broadcasts are a necessary part of local LAN traffic, but they are not required on other LANs and can even overwhelm slower WANs. Routers can generate broadcasts themselves if necessary (for example, to send a routing protocol advertisement), but do not pass on a received broadcast.
Routing operation is discussed further in the "Routing" section, later in this chapter.
NOTE The concepts of unicast, multicast, and broadcast apply to Layer 2 and Layer 3 separately. Although a router does not forward any type of frame, it can forward a unicast, multicast, or directed broadcast packet that it received in a frame. A switch, however, can forward a unicast, multicast, or broadcast frame.
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