LAN Segmentation
LAN segmentation simply means breaking one LAN into parts, with each part called a segment. The term LAN segment comes from the original use of a physical bus with 10BASE2 and 10BASE5. A single Ethernet segment consisted of the devices connected serially with coaxial cable. When 10BASE-T came along, the term segment still was used, this time to identify a hub with multiple devices connected to it.
Figure 9-7 repeats an earlier figure, but with the term segment noted on the figure.
Figure 9-7 Segments and Segmentation with a Bridge
Figure 9-7 Segments and Segmentation with a Bridge
With a single hub, or multiple hubs but no bridges, switches, or routers, you have a single segment. When you separate the network with a bridge, switch, or router, you create multiple segments. So, in Figure 9-7, you can say that the bridge separates the network into two separate segments. Many people use the term Ethernet segment very loosely, but for the CCNA exams, you can think of segment as meaning the same thing as collision domain.
As mentioned earlier, a collision domain is the set of LAN interfaces whose frames could collide with each other, but not with any other devices in the network. The bridge in Figure 9-7 creates two separate Ethernet segments, and each is a separate collision domain. Figure 9-8 shows a typical example of the definition of collision domains.
Each separate segment, or collision domain, is shown with a dashed-line circle in the figure. The switch on the right has a separate collision domain for each port. The hub near the center of the network does not create multiple collision domains because it repeats all frames out all ports. Routers also separate LANs into separate collision domains (although that was not covered earlier in this chapter.)
Just like the concept of a collision domain relates to where a frame can be sent and where it can cause collisions, the concept of a broadcast domain relates to where broadcasts can be forwarded. Bridges and switches forward broadcasts and multicasts on all ports. Because broadcast frames are sent out all ports, a bridge or switch creates only a single broadcast domain. A broadcast domain is the set of devices for which, when one of the devices sends a broadcast, all the other devices receive a copy of the broadcast.
Only routers stop the flow of broadcasts. Figure 9-9 provides the broadcast domains for the same network depicted in Figure 9-8.
Broadcasts sent by a device in one broadcast domain are not forwarded to devices in another broadcast domain. In this example, there are two broadcast domains. For instance, the router will not forward a LAN broadcast sent by a PC on the left to the segment on the right. In the old days, the term broadcast firewall described the fact that routers did not forward LAN broadcasts.
Figure 9-9 Broadcast Domains
Figure 9-9 Broadcast Domains
General definitions for a collision domain and a broadcast domain are as follows:
■ A collision domain is a set of network interface cards (NICs) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.
■ A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC will be received by all other NICs in the same broadcast domain.
In short, hubs do not actually segment an Ethernet into multiple segments. Bridges and switches do segment an Ethernet into different collision domains, and routers segment an Ethernet into different collision and broadcast domains.
The INTRO exam actually might phrase questions in terms of the benefits of LAN segmentation instead of just asking for the facts related to collision domains and broadcast domains. Table 9-4 lists some of the key benefits. The features in the table should be interpreted within the following context: "If I migrated from a single Ethernet segment to a network with two segments separated by a bridge/switch/router, and if traffic loads and destinations stayed constant, the result would be_."
|
Feature |
Bridging |
Switching |
Routing |
|
Greater cabling distances allowed |
Yes |
Yes |
Yes |
|
Decrease in collisions |
Yes |
Yes |
Yes |
|
Decreased adverse impact of broadcasts |
No |
No |
Yes |
|
Decreased adverse impact of multicasts |
No |
No* |
Yes |
|
Increase in bandwidth |
Yes |
Yes |
Yes |
*Switches today support several methods to optimize multicast forwarding, such as Internet Group Management Protocol (IGMP) snooping
*Switches today support several methods to optimize multicast forwarding, such as Internet Group Management Protocol (IGMP) snooping
Continue reading here: The Need for Spanning Tree
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