Hub

Nerd Cafe

Context:

• Four PCs are connected via a hub.

• The PC with IP 192.168.1.1 pings 192.168.1.4 (ICMP Echo Request).

• A hub is a Layer 1 device: it broadcasts any incoming frame out all other ports.

• The images show this process in detail.

1. Initial Topology

• All devices are connected to the hub.

• Devices and IPs:

  • PC1: 192.168.1.1

  • PC2: 192.168.1.2

  • PC3: 192.168.1.3

  • PC4: 192.168.1.4

• At this point, no traffic is being sent yet.

• The network is idle and ready.

2. PC1 sends ICMP Echo Request

• PC1 (192.168.1.1) creates an ICMP Echo Request packet.

• This packet is encapsulated in an Ethernet frame.

• Destination MAC: Broadcast or PC4’s MAC (depending on ARP already resolved or not—but hub doesn’t care).

• PC1 sends the frame into the hub.

3. Hub receives the frame

• The hub receives the signal on the port connected to PC1.

• The hub doesn’t inspect the packet at Layer 2 (it doesn’t learn or filter).

• The hub’s role is simple repetition/amplification of the signal.

4. Hub forwards to all ports

• The hub forwards the signal to all other ports except the one it received from.

• Here:

  • Signal goes to PC2.

  • Signal goes to PC3.

  • Signal goes to PC4.

• This is flooding behavior typical of hubs.

5. All PCs receive the frame

• All connected PCs receive the Ethernet frame.

• But only PC4 (192.168.1.4) will process it:

  • Checks the destination IP.

  • Matches its own.

  • Processes the ICMP request.

• PC2 and PC3:

  • See it’s not for them.

  • Discard the frame.

6. PC4 prepares ICMP Echo Reply

• PC4 creates an ICMP Echo Reply packet.

• Encapsulates it in an Ethernet frame with:

  • Destination MAC of PC1.

  • Source MAC of PC4.

• PC4 sends the frame into the hub.

7. Hub forwards reply to all other ports

• Hub receives the frame from PC4.

• Hub forwards it to all other ports except PC4’s port:

  • Sends to PC1.

  • Sends to PC2.

  • Sends to PC3.

• Result:

  • PC1 receives the ICMP Echo Reply.

  • PC2 and PC3 see the frame, see it’s not for them, and discard it.

Summary of Hub Operation in This Scenario

• The hub always forwards all frames to all ports (except the incoming one).

• No learning, no filtering.

• Inefficient compared to a switch.

• But it ensures connectivity in simple networks.

• In your ping scenario:

  • Both the request and the reply are broadcast (in terms of the hub’s behavior) to all connected devices.

  • Only the intended recipient processes the packet.

Cisco Packet Tracer Topology (Version 6.2.0.0052)

66KB

hub.pkt

Open