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What is Broadcast domain?

The range that a broadcast frame reaches. Switches and hubs forward broadcasts, so they belong to the same range, and routers (or routing on an L3 switch) mark its boundaries. With VLANs, a single switch can be divided into a separate broadcast domain for each VLAN. If the range is too large, broadcasts such as ARP increase and network efficiency drops.

Japanese name: ブロードキャストドメイン

Broadcast domain appears in 2 practice questions across 2 topics, including Computer Networking Basics, Network Design and Operations.

Field: Network Design and Operations

Try it: questions about Broadcast domain

On one L2 switch, VLANs are configured so that ports 1–4 are in VLAN 10 and ports 5–8 are in VLAN 20. Which statement about its operation is correct?

  1. ABecause VLAN 10 and VLAN 20 are on the same switch, they can communicate directly without a router
  2. BSplitting into two VLANs halves the speed of each port
  3. CBroadcasts sent by devices in VLAN 10 do not reach VLAN 20 ports, and traffic between VLAN 10 and VLAN 20 needs a router (or L3 switch)
Show answer and explanation

Answer: C. Broadcasts sent by devices in VLAN 10 do not reach VLAN 20 ports, and traffic between VLAN 10 and VLAN 20 needs a router (or L3 switch)

A VLAN (virtual LAN) creates logically separate networks on the same physical equipment, typically on a per-port basis. Each VLAN has its own broadcast domain, the range a broadcast reaches, and the switch forwards frames only between ports in the same VLAN. Traffic between different VLANs, such as VLAN 10 and VLAN 20 in this example, must therefore pass through a router or L3 switch that relays it at layer 3. When VLANs span several switches, a tag (VLAN ID) defined by IEEE 802.1Q is inserted into frames to indicate which VLAN they belong to. VLANs are used to separate networks by department, or to isolate a guest wireless LAN from the internal network.

An L2 switch with four PCs is connected to one interface of a router. A repeater hub with three PCs is connected to the router's other interface. No VLANs are used. Counting a collision domain as the range in which frames can collide on half-duplex Ethernet, which is the correct pair of numbers of broadcast domains and collision domains?

  1. A2 broadcast domains, 2 collision domains
  2. B2 broadcast domains, 6 collision domains
  3. C6 broadcast domains, 2 collision domains
Show answer and explanation

Answer: B. 2 broadcast domains, 6 collision domains

A router does not forward broadcasts, so each router interface is a separate broadcast domain, giving two here. An L2 switch makes each port a separate collision domain, so on the switch side the four PC ports and the port to the router are five separate collision domains. A repeater hub simply repeats signals to all its ports, so the hub, its three PCs and the router interface form one collision domain. That gives 5 + 1 = 6 in total. Today switches are normally connected in full duplex, and collisions cannot occur at all on a full-duplex link.

Broadcast domain in the explanations

Questions that mention Broadcast domain (2)

  1. On one L2 switch, VLANs are configured so that ports 1–4 are in VLAN 10 and ports 5–8 are in VLAN 20. Which statement about its operation is correct?
  2. An L2 switch with four PCs is connected to one interface of a router. A repeater hub with three PCs is connected to the router's other interface. No VLANs are used. Counting a collision domain as the range in which frames can collide on half-duplex Ethernet, which is the correct pair of numbers of broadcast domains and collision domains?

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Related terms

Router VLAN LAN (local area network) Wi-Fi Ethernet ACL (access control list) Administrative distance BGP (Border Gateway Protocol) Link aggregation Longest prefix match

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