I am studying up for CCENT, and I'm having some trouble understanding what a basic router does. I understand one of the main functions is to split up broadcast domains. When they say it splits broadcast domains, does that mean it only splits the WAN and the LAN? Can it split broadcast domains on LAN ports as well?

  • Let me answer your question with a question: If, for example, every device in the world has a unique MAC address, why bother with IP addresses, etc? You could, in theory, address any device in the world directly. Why do we need to route? – Ron Trunk Jun 20 '16 at 19:16
  • Splitting up broadcast domains is a side-effect of routers. – Ron Trunk Jun 20 '16 at 19:19
  • Well I understand the need for routing at Layer 3 it was a little unclear to me when Routers split up broadcasting domains on LAN ports. Especially when I see a depiction of an ISP router in the Text which explains there's actually a switch built into the router, so the router really only has 1 WAN and 1 LAN. A router with multiple LAN ports (not an ISP router) was a little unclear whether the router still split broadcast domains on those ports. – allegory Jun 20 '16 at 19:23
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    "LAN" and "WAN" ports are just names, usually based on the media type. To the routing "function," it doesn't matter what the media is. Routers can have hundreds of ports, some can be called WAN and some can be LAN. – Ron Trunk Jun 20 '16 at 19:28

The function of routers is to route traffic between different networks at layer-3. This can be any combination of WAN or LAN ports.

A router strips off, and discards, the layer-2 frames from traffic received on its interfaces. A router will then use the information in the layer-3 packet to switch the packet to the next interface in a path toward the destination. It will then create a new layer-2 frame for any traffic sent out one of its interfaces. That is why it breaks up layer-2 broadcast domains; the layer-2 information doesn't survive when the layer-2 frame comes into the router.

Some routers will allow subinterfaces and can understand and place VLAN tags on the frames. The router could be configured to route between the subinterfaces, allowing layer-3 traffic to be sent between the layer-2 broadcast domains.

  • So any combination of WAN or LAN ports can be split up with different broadcast domains, got it. – allegory Jun 20 '16 at 19:28
  • It's any router interface. – Ron Maupin Jun 20 '16 at 19:28

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