Broadcast Domains

CCNA Broadcast Domain Understanding broadcast domains is an essential part of the CCNA program. Broadcasts use bandwidth, which reduces available bandwidth to other services. However, unlike collisions, it is not good to eliminate broadcasts.

What is a broadcast? In network terms, a broadcast is a request that is sent to all devices on a LAN segment. The most common example is that a computer needs a service from another system to do something, but does not have the address of the host that can provide the service. The requesting computer will broadcast a message that basically say “Can anyone _____________ (fill in the blank with the needed service)?” The broadcast is read by every computer on the network segment, and the host that can provide the service will respond. To designate that the frame is a broadcast, the destination address is all ‘1’s” or hexadecimal FFFF-FFFF-FFFF. This is the key to all other computers that they need to examine the frame to see if it is for them. A broadcast domain is the set of computers that will receive a broadcast. The following are two examples of broadcasts or service requests. There are many other types of broadcasts, but these are the most common uses of broadcasts. 1. DHCP (Dynamic Host Configuration Protocol. 2. An ARP request for a MAC address that corresponds to an IP address. (Address Resolution Protocol)

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CCNA Broadcast Domain 1. DHCP (Dynamic Host Configuration Protocol). DHCP allows computers to get all of their IP information and configuration automatically. DHCP provides a simple method of enabling IP. First, the PC configuration (Taken from Start>Control Panel>Network Connections>Local Area Connection>Properties>Internet Protocols (TCP/IP).

A PC configured to obtain an IP address automatically (use DHCP) will request a DHCP configuration from a DHCP server. When the PC boots up, it does not know the MAC address of the DHCP server, but it is configured to send a DHCP request. Since it does not know anything about the DHCP server, it has to learn who the server is. So it sends a broadcast that basically says, “Can anyone (FFFF-FFFF-FFFF, all “1’s”) give me an IP configuration?” The destination address identifies the packet as a broadcast. Any packet with all “1’s” or hexadecimal “FF’s” in the destination address field is a broadcast frame to be read by every computer in the network. The DHCP server answers “I can give you a DHCP configuration.” The computer that requested the DHCP service now knows the MAC address of the DHCP server. The DHCP server knows the MAC address of the requesting computer. The computers can have a dialog that results in the first computer receiving the information that it needs. The following slides will show how broadcasts are used. After reviewing slides 3-11, determine which broadcasts are unnecessary in this network.

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CCNA Broadcast Domain This is our network.

Server MAC: 3333-3333-3333

Gateway

Gateway router MAC: 4444-4444-4444

Our PC MAC: 5555-5555-5555

DHCP Server MAC: 2222-2222-2222

Our PC, MAC Address 5555-5555-5555, is turned on, boots up, and reads the configuration that tells it to request an IP configuration from a DHCP server. Our PC does not know which devices is a DHCP server, it only knows to request a DHCP configuration.

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CCNA Broadcast Domain Server MAC: 3333-3333-3333

Gateway router MAC: 4444-4444-4444

Can anyone service DHCP requests? Gateway

Our PC MAC: 5555-5555-5555

DHCP Server MAC: 2222-2222-2222

The actual frame would look something like this: FFFFFFFFFFFF Broadcast: addressed to all devices on the network

555555555555 Sent by Our PC 555555555555

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Can anyone service DHCP requests?

FCS

DHCP request

All computers in the broadcast domain, or on the segment, would receive the broadcast and examine the request. All of the computers except the DHCP server would ignore (discard) the request, since they don’t do DHCP. The DHCP server would recognize that it needed to provide an IP configuration. It knows who requested the IP information to: 555555555555. The DHCP server would look in an assignment table for an IP address that was not being used, and then check the availability with a broadcast ping to the IP address that will be assigned to Our PC.

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CCNA Broadcast Domain Server MAC: 3333-3333-3333

Gateway router MAC: 4444-4444-4444

Is anyone using IP address x.x.x.x Gateway

Our PC MAC: 5555-5555-5555

FFFFFFFFFFFF Broadcast: addressed to all devices on the network

222222222222 This is from 2222-2222-2222

80

DHCP Server MAC: 2222-2222-2222

Ping IP address x.x.x.x

FCS

Does anyone have this IP address?

If another computer in the broadcast domain is using the IP address, it would respond to the broadcast from the DHCP server, and the DHCP server would chose another IP address that is (hopefully) not being used. It would repeat the process, using broadcasts (FFFFFFFFFFFF) until it found an unused IP address.

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CCNA Broadcast Domain

If no computer responds to the ping, the DHCP server would send the requested DHCP information to our computer using address 5555-5555-5555, the MAC address of Our PC. The data is from the MAC address of the DHCP server: 2222-2222-2222

Server MAC: 3333-3333-3333

Gateway router MAC: 4444-4444-4444

Here is your IP configuration information Gateway

Our PC MAC: 5555-5555-5555

555555555555 This is a reply to 555555555555

222222222222 This is from 222222222222

80 Here is your IP information

DHCP Server MAC: 2222-2222-2222

FCS

DHCP response

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CCNA Broadcast Domain Our computer is now happy that it has an IP address, and acknowledges that it has the information it needs.

Server MAC: 3333-3333-3333

Gateway router MAC: 4444-4444-4444

Thanks for your support Gateway

Our PC MAC: 5555-5555-5555

222222222222 This is a reply to 222222222222

555555555555 This is from 555555555555

80 Thank you very much

DHCP Server MAC: 2222-2222-2222

FCS

DHCP acknowledgment

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CCNA Broadcast Domain ARP (Address Resolution Protocol) Request When a computer communicates with an IP address in the same network, it does not use the IP address. Instead it uses the MAC address of the other computer. If the IP address is remote (not in the same network or subnet), it sends the data to its default gateway or router, which must be in the same network or subnet. The default gateway is always defined as an IP address, not a MAC address. To send the data to an IP address in the same network or subnet, Our PC must learn the MAC address associated with the IP address. Computers use a process called ARP: Address Resolution Protocol. ARP uses broadcasts to learn the MAC addresses on computers associated with specific IP address. In this example, Our PC needs the MAC address of the gateway computer, so it issues an ARP request, a broadcast request for the MAC address associated with an IP address. Server MAC: 3333-3333-3333 Server Gateway router MAC: 4444-4444-4444 Broadcast: Who has IP address X.X.X.X? Gateway

DHCP

DHCP Server MAC: 2222-2222-2222

Our PC MAC: 5555-5555-5555 FFFFFFFFFFFF Broadcast: This is to everyone

555555555555 This is from 555555555555

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Who has IP address x.x.x.x?

FCS

ARP request

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CCNA Broadcast Domain The gateway router says to himself, “Hey, that’s my address. I need to tell 5555-5555-5555.” He replies to the ARP request

Server MAC: 3333-3333-3333

Gateway router MAC: 4444-4444-4444

Reply: I have IP address x.x.x.x Gateway

Our PC MAC: 5555-5555-5555

555555555555 This is the reply to 555555555555

444444444444 This is from 444444444444

80

I have IP address x.x.x.x

DHCP Server MAC: 2222-2222-2222

FCS

ARP reply

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CCNA Broadcast Domain Our PC gets the MAC address of the router, and sends its IP data to the router.

Server MAC: 3333-3333-3333 Gateway router MAC: 4444-4444-4444 Here is my data. Gateway

Our PC

DHCP

DHCP Server MAC: 2222-2222-2222

Our PC MAC: 5555-5555-5555

444444444444 Broadcast: This is to everyone

555555555555 This is from 555555555555

80 Here is my data

FCS

Data

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CCNA Broadcast Domain The previous slides showed an Ethernet network as if it was a coax segment. Coax was replaced by hubs, but the rules for broadcasts still apply. In a hub network, a broadcast is sent to all hubs, and is transmitted on all ports. A PC on one hub will communicate with a server or router on another hub as if it was on the same hub. Bandwidth is used on all hubs to service the request.

HUB

HUB

Broadcast

Broadcast

Broadcast

Our PC

Broadcast

HUB

Broadcast

Server Gateway

Broadcasts: What’s the big deal? Just cut them all out. Broadcasts are an essential part of a network. Without broadcasts, every part of the network would have to be defined in every system before it could be used. That is a lot of work, and would probably require changing the operating systems, which is a major problem. We cannot run a network without broadcasts. The problem is that broadcasts take up bandwidth. When a computer transmits a broadcast or replies to a broadcast, no other computer can use the network. Both the broadcast and the reply take bandwidth from the network. Anything that can be done to control broadcasts increases bandwidth. Broadcasts are a good thing. Bandwidth is better.

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CCNA Broadcast Domain HUB

Broadcast

Broadcast

Our PC

HUB

Broadcast

Broadcast

HUB

Broadcast

Server Gateway

Consider the network above. Suppose the network is large, as in very large. As networks grow, the bandwidth required by PCs just for broadcasts grow as well, consuming a lot of bandwidth. As the number of broadcasts increases, the bandwidth available for data decreases. The result, less bandwidth and slower response. Definition: Broadcast domain A broadcast domain can be defined in several ways, and each of them is correct. 1.A broadcast domain consists of all of the computers that will hear a broadcast. 2.A broadcast domain consists of all of the devices that can communicate at Layer 2. 3.A broadcast domain is a subnet. Rule: The larger the broadcast domain, the less bandwidth available for data.

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CCNA Broadcast Domain HUB

Broadcast

Broadcast

HUB

Broadcast

Our PC

Broadcast

HUB

Broadcast

Server Gateway

Routers do not route (or forward) broadcasts. A router limits (reduces) the size of a broadcast domain. Consider the network above. The only limit for broadcasts in this network is the router. If each hub represented a large number of computers, the broadcast domain would be very large. To reduce the size of the broadcast domain, more routers (or router interfaces) must be used. The following example shows a single router supporting each hub on a different interface.

HUB

Broadcast

HUB

Broadcast

HUB Broadcast

Gateway

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CCNA Broadcast Domain HUB

HUB

Broadcast

Broadcast

HUB Broadcast

Broadcast

Gateway

A network design like this one will limit the size of the broadcast domain. In this example, a large broadcast domain will be spit into three smaller broadcast domains. Since routers do not forward broadcasts, each hub becomes a single broadcast domain. The number of computers in each network is reduced, which decreases the number of broadcasts and increases the bandwidth available for data. The speed of data transmissions stays the same, but the broadcast data is reduced, thereby increasing available bandwidth. It is important to remember that broadcasts are required for most networks. Removing all broadcasts would seriously impact the network. The goal is to make the number of broadcasts manageable, not eliminate them completely.

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CCNA Broadcast Domain Layer 2 Switch

Broadcast

Our PC

Broadcast

Layer 2 Switch Broadcast

Broadcast

Layer 2 Switch Broadcast

Server Gateway

Layer 2 switches process broadcasts much like hubs do. The switch does not know which devices should receive a broadcast, so it forwards all broadcasts to all active ports. In the example above, a switch network connected by trunks will forward the broadcast to all active ports, including the trunk ports that connect the switches. Each switch, in turn, forwards the broadcast to all of its active ports. (NOTE: this is a simple explanation of switch function. Actually, Cisco switches support Virtual LANS. A VLAN is a group of switch ports that function as a separate broadcast domain. A Cisco switch can support multiple broadcast domains.) To reduce the size of the broadcast domain, each switch can be supported by a router interface.

Switch

Broadcast

Switch

Broadcast

Switch Broadcast

Gateway

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CCNA Broadcast Domain Broadcast domains vs. Collision Domains Originally, broadcast domains and collision domains were the same thing. This was the result of the media used to support the network, either coax or hubs. Layer 2 switches such as Cisco switches have dramatically changed collision domains, even to the point of completely removing collisions from a network, assuming that the network is designed correctly. Broadcast domains have changed very little. The requirements for broadcasts have not changed. Broadcasts remain a critical part of LAN function.

Copyright by Hedgehog Technical Institute® 2009 Broadcast Domain

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