Hub behind a switch

ksool · Posted: Sun Sep 03, 2006 6:37 am Post subject: Hub behind a switch

I've just added a hub behind a 4 port switch so that I could attach a few more machines. The hub is connected to the switch through an uplink port (no idea how this is different to just connecting to one of front ports).

My question is this: When the computers between the hub try to communicate with each other, the hub will send the packet out to the switch (or will the uplink port filter this?) and then the switch will send the packet it to every port since it won't be able to find the destination (or will it send it back to the hub since it recognizes that the destination is back where the packet came from?). Obviously, I don't want to send all those packets around my network unnecessarially.

Somewhere in there, there's a question.
And I'd be much oblidged if anybody could help clear this up.

jballou · Posted: Sun Sep 03, 2006 7:44 am Post subject:

Simply, a hub divides available bandwidth among all acive ports because each packet is broadcast to every port, even unicast traffic.
A switch is smarter, it keeps MAC addresses in a table and sends it to whatever port that MAC address last replied on.
So if you have computers named hub1 hub2 and hub3 on the bub, and switch1 switch2 and switch3 on the switch, if any of the hub{1,3} hosts send any traffic each of the other hub computers hear it. If it's destined for the switch, it also hears it, but only the switch{1,3} computer is it destined to will hear it. If switch1 wants to talk to hub1, the switch sends it out the hub port, then all the hub computers hear the same packet.

HTH
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ksool · Posted: Sun Sep 03, 2006 4:06 pm Post subject:

Ah thanks, but what I'm thinking is the following, when the hub computer send each other a packet, say, from hub1 to hub2, will that traffic also be sent to the switch? And then from my understanding, the switch won't know the location of hub2, and will send out that packet to all of the switched machines.

Is that correct?

Monkeh · Veteran Joined: 06 Aug 2005 Posts: 1656 Location: England

ksool · Posted: Sun Sep 03, 2006 4:17 pm Post subject:

I had read (wikipedia) that if a switch doesn't know where to send a packet, it sends it out to all ports.
The question is, is it aware of the machines behind the hub. If so, I would assume it would discard it.

think4urs11 · Posted: Sun Sep 03, 2006 5:31 pm Post subject:

jhmartin · Tux's lil' helper Joined: 03 Sep 2003 Posts: 95