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Why is it, that on L2 (when there is no MAC Table entry for a new packet) there is MAC Flooding. But at the same time on L3 there is an ARP request (when there is no entry in the ARP table for a new packet) and no flooding?

Jimmy88
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1 Answers1

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It's not clear to me what the "intention" of your question is, but let me make a few points:

A switch floods ethernet frames to all switch ports when it doesn't have a MAC address to switch port mapping for the destination MAC address.

ARP resolves IP addresses to MAC addresses. A switch doesn't perform ARP unlesss the switch itself is communicating with a host on the network.

When a host performs an ARP for an ip address it broadcasts that ARP request to the local network.

Flooding and broadcasting are technically two different things.

So, when a host performs ARP for another host, you should see that ARP request broadcasted on the local network.

joeqwerty
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  • thanks for your answer! I know what these two mechanisms are but I am wondering why they are like this. Why is it, that on L3 we use a specific protocol to map ip <=> mac, when on L2 we simple flood to map mac <=> interfaceport. – Jimmy88 Apr 29 '15 at 14:34
  • @Jimmy88 because in the hub-age of networking (and before) there were very few switches. Generaly every packet went everywhere. So there was no need for another protocol. And it's way simpler to implement (especially important for small dumb $10 switches; back in the days for every switch) than separate protocol – Fox Apr 29 '15 at 14:46
  • okay, i understand. but what is the reason that on L3 there is no such flooding mechanism? because of network performance and congestion? – Jimmy88 Apr 30 '15 at 08:03