Switches – Why Don’t They Always Flood?

ethernetpacket-pathswitch

If a switch doesn't have a destination in its table it floods to all (but the sending) ports. Why doesn't it always do this if it can, since it would be faster than doing a look up in a table? I guess it has to do with collision: when 2 nodes transmit on the same medium simultaneously. But collisions can always happen anyways, and can be fixed using CSMA. I guess CSMA is slow and wants to be avoided?

It would be nice if we had tags for hubs and flooding.

Best Answer

cpt_fink and Shane already gave good answers, but I will add $0.02...

Let's assume you flood every frame that comes into a switch, you're essentially turning what we know as switches into a hub. That causes problems with:

  1. Security / Privacy: IP networks are largely unencypted. Flooding packets shares private information with everyone in that broadcast domain... DNS lookups, FTP passwords, websites you're visiting. All I need is wireshark on a flooded network, and I know a lot about what is happening. It's much easier to hijack TCP sessions if you know the TCP sequence numbers in use.
  2. End user count: If we flooded packets for every user to every other user, there is a much lower limit for how many users we can pack into one switch, because every user consumes the other user's bandwidth (see next bullet). Ultimately, this means you're spending a lot more money on network administrators, network infrastructure, vendor support costs, power, rack space, cooling, etc...
  3. User throughput: If any user on the switch sends line-rate traffic (i.e. no faster than 1Gbps on a GigabitEthernet link), we are flooding that line-rate traffic to every other user in the switch (as well as the broadcast domain). This is terribly inefficient. Mac-learning is much better than flooding

FYI: YLearn provided some practical examples of how flooding causes problems in wired networks.

A final thought:

If switches always flooded instead of learning mac-addresses, switched topologies would no longer be viable in modern networks. Engineers would resort to routing as quickly as possible because the three issues listed above are deal-killers for many wired network designs.

Interestingly enough, wireless 802.11 only implements privacy and security from the list above; otherwise wifi has the aforementioned problems with "flooding" non-broadcast traffic to all hosts (technically it isn't flooding, but the result is the same). 802.11's privacy and security measures make wifi at least a viable option.

EDIT:

In a comment, Celeritas said:

... you say that switches are more secure, but I seriously doubt that people choose a switch over a hub because they want security, security would be built at a different level. Also it would be nice if the answers didn't use too much jargon, for example what is "linerate"?

I am afraid you have misunderstood why people choose switches over hubs. People choose switches instead of hubs, because all three disadvantages listed above are a problem for wired networks.

802.11 wifi has two of the three problems that hubs have; however, the privacy and security measures built into wifi make it a feasible choice in some situations.

Apologies for confusing you with the term "line-rate". Line-rate means you're sending traffic as fast as the wired NIC can. On gigabit ethernet links, line-rate depends on the packet size you send, but line-rate cannot exceed the transmit speed of a GE NIC, which is 1,000,000,000 bits per second.

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