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Consider, there is router R connected to switches A,B,C. Each switch is connected to 5 systems. If a system in switch A sends a limited broadcast, will it be broadcasted only to the systems under switch A, or to all the systems under switches A,B,C.

Please explain the flow of packets at each stage. i.e. Does it go from Switch to Router to Switch, or just Switch to Switch, etc.

If a router simply drops the packet, then how does it get broadcasted to other switches in that subnet?

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  • homework? we don't do homework... Nov 30 '15 at 11:56
  • @CraigConstantine Haha.. I'm not a student anymore. Nov 30 '15 at 13:16
  • @CraigConstantine Please tell me in which section of stackexchange does this question fit into. Nov 30 '15 at 13:17
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Broadcasts will not cross a layer-3 boundary (router). If the switches are connected to the router in separate subnets, the broadcast will be limited to a single switch. If the switches are bridged at layer-2, the broadcast will be propagated to any switch in the bridge group.

Basically, if traffic from one switch to the other must be routed (different subnets), broadcasts will be limited to a single switch.

If the switches only connect to the router at layer-3, broadcasts will be limited to a single switch. If the switches are connected to the router in a bridge group, the router acts like another layer-2 device for any traffic between the switches, and will pass layer-2 traffic (like broadcasts) between the switches. If the switches are connected together, layer-2 traffic will pass from one switch to another switch.

You need to define the layer-2/layer-3 boundaries in order to properly answer this question. Once you do that, you can tell where a broadcast will travel since it will not cross such a boundary.

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  • Wow! I didn't know that switches can be connected to the router in separate subnets. Nov 29 '15 at 19:59
  • It depends on how many router interfaces you have. Routers usually have a limited number of interfaces, and router interfaces cannot normally be in the same subnet. A layer-3 switch can route also, and you can create layer-3 virtual interfaces (SVIs), each of which will be in a separate subnet.
    – Ron Maupin
    Nov 29 '15 at 20:03

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