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I am new to networking and I'm trying to learn some data center architecture, but theres one thing i don't understand. If you have a redundant switch and router setup, with active active.

Like this:

data center archetecture example

If the server is connected to both switches, for redundancy. Then why wont two instances of the same frame go on to the Internet?

Gerald Schneider
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Elias Fyksen
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  • Because the frame is transmitted only once, out of one of the interfaces connected. It would not be transmitted from the server to both switches.If you placed a HUB between the server and the two switches (Connecting both NICs of the server to the hub and then a connection to each switch from the HUB, then it would reach both switches). – Mikael Dyreborg Hansen Jun 16 '17 at 06:29
  • @MikaelDyreborgHansen that's not quite correct. A switch would forward the frame if recognize the layer 2 destination, a hub would forward it unconditionally. But to the original question (and I'm not sure why there's a diagram of a web farm here), the frame, datagram, packet, whichever, they are *discrete*. When one goes out, it's just one. – quadruplebucky Jun 16 '17 at 06:38
  • @quadruplebucky That was my point exactly. If the server was connected to a HUB and the HUB was also connected to the switches, the hub would then transmit the packet from the server to both switches. A little irrelevant however as HUBs (to my knowledge) are not used anymore (At least shouldn't be in my oppinion :) ) – Mikael Dyreborg Hansen Jun 16 '17 at 06:56
  • ?? Wouldn't make a bit of difference to the switches or the logical topology. – quadruplebucky Jun 16 '17 at 07:01
  • Yes it would. The hub transmitt to both switches, which would put two frames on the network instead of one – Elias Fyksen Jun 19 '17 at 20:07

1 Answers1

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Because the packets aren't sent over both links. A connection between two endpoints on both sides of the switch always uses only one link. Which one is usually determined by a hashing algorithm. If a link goes down the hashing algorithm sends the packets over the remaining links.

The hashing algorithm (there are multiple that can be used) can take multiple factors into account (common are source and destination IP and port, but there can be more). This ensures that answer packets reach the source via the same link the outgoing packets used and adds load balancing as another benefit.

For more details a good keyword for an internet search is link aggregation.

Gerald Schneider
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