Cisco CCENT / CCNA Certification Exam Training :
Broadcasts And Hubs, Repeaters, Switches, And Routers
By Chris Bryant, CCIE #12933
Yesterday’s Cisco CCENT certification exam training tutorial was a definition of broadcasts, their use in today’s networks, and why we have to be wary of a broadcast storm. Today, we’re going to discuss how four common network devices – hubs, repeaters, routers, and switches – affect broadcasts that are traveling over our network.
In some cases, these devices have no effect at all! Hubs and repeaters have no effect on broadcasts, unicasts, or multicasts. Hubs and repeaters work at the Physical layer of the OSI model, and as such they’re not concerned with switching or routing. Hubs and repeaters have one basic purpose, and that is to strengthen the actual electronic signal traveling over the network cables. They do not route, they do not switch, and they do not know one kind of transmission from another.
A network device that does know a broadcast from a unicast is our major Layer 3 device, the router. Routers run at the Network layer of the OSI model, and a router’s default behavior is not to forward a broadcast at all. Note that a router will receive and accept a broadcast, and may even generate one, but routers do not forward broadcasts.
A switch will also receive and accept broadcasts, but that’s where the switch’s behavior is different from a router’s behavior. A switch will actually forward a copy of a broadcast frame out every single port on that switch except the port that received the broadcast in the first place. We don’t get to say “never” in networking very often, as there always seems to be an exception to the rule, but a broadcast frame will never be sent out the same port it came in on – bottom line.
The problem with this behavior is that we probably end up with a lot of unnecessary broadcasts being forwarded by the switch. That’s a load on the switch’s CPU and memory, and the hosts connected to the other ports end up processing packets they don’t need. This broadcast propagation ends up using some of our valuable bandwidth as well, and we can’t afford to waste bandwidth!
There’s good news, though – Cisco switches allow us to create “virtual local area networks”, or VLANs, to prevent broadcasts from being sent to hosts that do not need them. Creating a VLAN is simple, and we’ll take a look at how to create VLANs in the next installment of my exclusive Cisco CCENT / CCNA certification exam tutorial series!
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CCIE #12933
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