Broadcast Domain

In networking, a broadcast means that we send something that everyone receives, whether they need/want it or not. Switches will forward broadcast traffic on all their interfaces, except the one they received the broadcast on. Here’s an illustration to visualize this:

switches forward broadcast traffic

H2 on the left side sends a broadcast which is forwarded to all other switches and computers. There are a number of applications and protocols that use broadcast traffic. A good example is ARP (Address Resolution Protocol). Switches will recognize it as broadcast traffic by looking at the destination MAC address. Here’s an example of an ARP request:

wireshark-capture-arp-request

Above you can see that the destination MAC address is FF:FF:FF:FF:FF:FF.

Switches will also broadcast Ethernet frames if they haven’t learned the destination MAC address yet.

A broadcast domain is a collection of network devices that receive broadcast traffic from each other. For example, here’s our network with three switches again:

switch single broadcast domain

Broadcast traffic is not very efficient. For example, let’s say that that ARP request is sent by H2 to figure out the MAC address of H1. The information in the ARP request is useless for SW2, SW3, and H3 but they do receive it. SW2 and SW3 have to waste bandwidth forwarding the broadcast frame and H3 has to waste some CPU cycles to look at an ARP request that it doesn’t care about.

Nowadays, CPUs are fast and we use fast (Gigabit) interfaces so the impact of broadcast traffic is not so bad. About twenty years ago, having a network with a lot of devices could impact your performance because of broadcast traffic. Older operating systems also used a lot of broadcast traffic for their applications.

Broadcast domains with 1-1000 devices are common nowadays.

Still, a single device that sends a LOT of broadcast traffic does affect the entire broadcast domain so it’s a good idea to limit the size of your broadcast domains.

How do we create multiple broadcast domains? One way to do this is by adding a router to your network:

Routers breaks broadcast domain

Routers, unlike switches, do not forward broadcast traffic. When H2 sends that broadcast frame, it will be forwarded by SW2 to R1 but that’s it. This does mean that traffic from H2 to H1 will have to be routed but that’s another story. Here are the broadcast domains we have now:

router breaks broadcast domain

We now have three broadcast domains, one on each side of the router.

Is there any option to create multiple broadcast domains on a switch? There sure is, it can be done with VLANs (Virtual LAN).

Normally on a Cisco switch, all interfaces belong to VLAN 1 which means all interfaces are able to communicate with each other. With a VLAN, we can separate this. For example, we can put interface 1-9 in one LAN and interface 10-19 in another LAN. These two VLANs won’t be able to communicate with each other, creating two different broadcast domains.

We will take a closer look at VLANs in another lesson but for now, keep in mind that it’s a possibility:

single switch two broadcast domains

Conclusion

You have learned what a broadcast domain is:

  • A broadcast domain is a collection of devices that receive broadcast traffic from each other.
  • Switches will forward broadcast traffic to all interfaces, except the one where it originated from.
  • A lot of broadcast traffic might impact your network performance so reducing the size of the broadcast domain is something to consider.
  • Routers do not forward broadcast traffic, they break broadcast domains.
  • VLANs on switches also allow you to break broadcast domains.

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Forum Replies

  1. Hi Rene,

    I learned from http://www.netcontractor.pl/blog/?p=184 that control traffic from Layer 2 protocols like ( DTP, VTP , CDP , PAgP , STP, etc ) use VLAN 1.

    And I make sure of that by doing this experience :

    - I made a simple topology of connection two switches and making connectivity as trunk.
    - I made one switch VTP server and another as VTP client.
    - I also setup RSPAN to monitor the packets.
    - Results, I saw VTP, CDP traffic marked with VLAN ID 1.
    - Then, I made another vlan and disallowed Vlan 1 in the trunk.
    - Results still the same.
    - I thought it

    ... Continue reading in our forum

  2. Hi Daniel,

    There’s a “technical” and “practical” aspect to this question :slight_smile:

    Let’s start with the technical part…a lot of networking people will tell you that you shouldn’t have > 200 hosts in a subnet since there will be too much broadcast traffic and it will slow down your network. This might be true 10 years ago but nowadays, your computers won’t be bothered much with broadcast traffic and it shouldn’t be an issue for your switches. You could probably put ~1000 hosts in a single subnet and not notice any performance issues.

    The more important issue (the practic

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  3. Hi I think I may have gotten a bad prep exam question as I cannot get it to work in labs and it does not make sense fully to me from how I learned VLANS. I will upload the practice test question from Boson and would like input please.

    //cdn-forum.networklessons.com/uploads/default/original/1X/05b2d7038e648c45e08123c84caf728638301b4b.JPG

    above is what they say is the answer and the topology its very brief. and below is their explanation.

    //cdn-forum.networklessons.com/uploads/default/original/1X/6cb467368403ad27ae72143183da8d340c120a6b.JPG

    Now I tried this in

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  4. Hello Justin

    Let’s say a switch has 24 access ports where ports 1-12 are on VLAN 10 and ports 13-24 are on VLAN 20. Let’s say a broadcast frame is sent on port 1. The switch will receive that frame and send it out of ports 2 to 12. Why? Because it knows that it entered port 1, therefore it is on VLAN 10, therefore it will send it out of all ports that have been configured on VLAN 10. It doesn’t even look at the details of the frame itself, because there is no data in the frame that gives the switch VLAN information. The information comes only from the fac

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  5. Laz,

    All of your posts are extremely well written. Thank you for explaining things much simpler than I can :slight_smile: Thanks,
    Scott

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