Network Design and Operations
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?
- A2 broadcast domains, 2 collision domains
- B2 broadcast domains, 6 collision domains
- 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.
- A. This counts the whole switch as one collision domain. A switch makes each port its own collision domain.
- B. Correct. The router splits the network into two broadcast domains, and there are five collision domains on the switch side and one on the hub side.
- C. This mixes up the numbers of broadcast and collision domains. Routers stop broadcasts; switches and hubs do not.