Introduction to Networking

Unit 6

Lesson 4 - Routers

Broadcast Domains

Earlier in this unit, we introduced the concept of a collision domain, which is a segment of a network in which devices compete for access to the shared transmission medium. Equally important is the idea of a broadcast domain, which is an area of a network in which broadcast frames are transmitted.

Broadcast traffic or background traffic consists of frames that carry the network's "administrative overhead." Broadcast frames are sent out for several reasons. For example, if a node has a packet to send and knows the destination network address but not the NIC address, it will broadcast a message asking for the node with the matching packet address to reply with its NIC address. In a TCP/IP network, this process is performed by the Address Resolution Protocol (ARP). Servers, routers, and printers also broadcast periodic messages, called "service advertisements," that announce their presence to other network devices.

These nonproductive (or semiproductive) transmissions typically represent 5 to 20 percent of overall network traffic. Thus, excessive broadcast traffic can significantly decrease the available bandwidth in a LAN.

Broadcast frames have a special destination address of all 0s. This special address tells all computers that receive the frame to process it. When a bridge or switch sees a frame addressed to a specific NIC address, it makes a switching decision. However, when a Layer 2 device sees a frame with a broadcast address, it has no choice but to send the frame out every port. Thus, broadcast traffic is received by all devices physically connected to each other or to a Layer 2 device. This area of the network is called a "broadcast domain" and is illustrated on the Ethernet Broadcast Domain Diagram.

Ethernet Broadcast Domain

In other words, a switch or bridge can segment a single large LAN collision domain into several smaller collision domains. However, the individual collision domains created by the switch are still members of the same broadcast domain, because a switch transmits broadcast traffic out all ports. This means that broadcast traffic originating in one collision domain is still forwarded to all other collision domains.

One way around this particular broadcast traffic issue (when using switches) is to program each switch and tell it where to send broadcast frames. This technique is referred to as creating "virtual LANs" or VLANs. The Introduction to Local Area Networks course discusses VLANs in more depth.

Routers for Broadcast Containment

To create separate broadcast domains, it is necessary to segment the network at Layer 3. A router effectively contains both regular network traffic and broadcast traffic within each network segment, and only directs intersegment traffic between network segments. This approach can improve the effective throughput of the entire network.

Router Advantages and Disadvantages

Routers offer several advantages over bridges and switches:

	Like a switch, a router provides users with seamless communication between individual LAN segments. Unlike a switch, a router forms the logical boundary between entire networks or groups of network segments.
	Routers provide efficient WAN access, because they do not forward broadcast traffic.
	A router can provide a firewall service, because it forwards only traffic specifically addressed to go across the router. Routers keep potentially disastrous events, such as broadcast storms, local to the area in which they occur, preventing them from spreading across the corporate network.
	The enhanced intelligence of a router allows it to support redundant network paths, and select the best forwarding path based on several factors in addition to the destination network address. This increased intelligence can also result in enhanced data security, improved bandwidth utilization, and more control over network operations.
	Routers can flexibly integrate different Data Link Layer technologies, such as Ethernet, Fast Ethernet, Token Ring, and FDDI. They can also consolidate legacy IBM mainframe networks with PC-based networks.

However, routers also have several disadvantages:

	The additional software processing performed by a router can increase packet latency, reducing the router's performance when compared to simpler switch architecture.
	To be "routable," an architecture must have a Network Layer. Not all architectures do, and those protocols must be bridged. "Unroutable" protocols include the DEC-LAT terminal communications protocol, IBM's SNA, and NetBIOS/NetBEUI.

Activities

See the Activities and Extended Activities section in Unit 6 Lesson 4 in your textbook Introduction to Networking to test what you have learned so far.

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