How Does LAN Traffic Get Routed On A Router?

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WildMonkey365

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Aug 30, 2016
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As many routers & firewalls I've installed I never truley understood how traffic gets routed internally from PC to PC. I am assuming that computers locating resources from something like a DNS server, network drive or a shared folder, the computer will need to access the resources by traversing the router/ default gateway correct? Do computers on a LAN access resources from each other directly without going through the router that they are connected to?
 
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The largest issue is defining what you mean by "router" when asking this question. It is fairly obvious when you have separate physical switch connected to a router with a cable. It is pretty easy to see you can unplug the cable between the router and the switch and the end devices will continue to function....at least until the dhcp times out.

When plug 2 cable into a router and ask this question you soon get into arguments about the definition of words. There are people that say since there is only 1 physical box then the traffic must flow thought the router and you quickly get into arguments about what the word "routed" means.

Years ago it was much harder because it was all done by software with the same cpu. What you find...
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nigelivey

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Dec 6, 2011
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Wireless and ethernet ports on a router are Layer 2, they do not pass over the LAN boundary therefore are not routed only switched. The switch part of the router or better described as a gateway device learn the MAC addresses of connected to a port and therefore know where to forward packets to.
 
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bill001g

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Aug 9, 2012
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The largest issue is defining what you mean by "router" when asking this question. It is fairly obvious when you have separate physical switch connected to a router with a cable. It is pretty easy to see you can unplug the cable between the router and the switch and the end devices will continue to function....at least until the dhcp times out.

When plug 2 cable into a router and ask this question you soon get into arguments about the definition of words. There are people that say since there is only 1 physical box then the traffic must flow thought the router and you quickly get into arguments about what the word "routed" means.

Years ago it was much harder because it was all done by software with the same cpu. What you find in current routers if you take them apart is a small switch chip connected to the lan ports. This chip works exactly the same as if you had a external switch. So you have a switch and router function in a single box. So in this case it traffic between the users on a lan only goes as far as this switching chip it does not pass to the router main cpu. Now to make this even more a mess lately they have moved this switching chip into the same physical chip as the main router. Logically it still functions as 2 separate units but it makes it even harder to answer the question.

This is why professionals tend to not use the term "router" as much. Most times traffic is referred to as layer 2 and layer 3 traffic to not get into arguments about certain hardware implementations.
 
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