Router connected to a switch. How does it work?

In article , John wrote: :If I plug in a :router to the switch, how does the switch know to route traffic for :all the computers connected to the router the requests?

I suggest you read my recent posting,

:Does it have :the ability to know that there are multiple mac addresses on one :network connection?

Yes, and it happens in nearly every LAN.

Initially it doesn't and has to send traffic to all ports, until it learns specific routes from the traffic. As soon as it sees traffic from a computer, it recognizes which port is associated with the computer's MAC address. At intervals, the cached routes will expire causing it to temporarily "forget" where a computer is connected. It is also possilbe to overflow the cache, if there's a huge number of devices on the local network. This will also cause the switch to "forget".

The host sending the packet knows its own IP address and its own IP network mask. And the host sending the packet also knows the IP address of the destination. Correct?

With this info, the host sending the packet knows whether that packet is destined inside the same IP subnet as its own, or whether the packet is destined to a different IP subnet.

If the packet is internal to the sender's own subnet, the sender simply broadcasts an ARP request to resolve the IP address to a local MAC address. And that's the end of it. The host also stores that IP-MAC address pair for a short period of time (say, 3 minutes or so), so it doesn't have to ARP so often. (These MAC addresses are stored in hosts long enough to prevent constant ARP broadcasts, but short enough so a change in the local network will not cause a long delay in getting frames moving again.)

But if the packet is destined to a different IP subnet, the sender knows to use the "default router" already configured in its routing table. (Sometimes other routers are also configured in hosts, aside from the default router, but that's just a detail you can ignore for now.)

So the sender knows to send packets heading out to another IP subnet to this default router. It has the IP address of the default router, so it will do an ARP request to find the MAC address of the default router, same as it would do for any other host, and off the packet goes. To the router, and on to other routers, until it reaches its destination.

I'm not sure this has anything to do with it. Yes, a switch can certainly keep track of multiple MAC addresses reachable on any given interface. But in the simple case, where there's just one switch in your local Ethernet and a router connected to one of its ports, the only MAC address associated with the router's port is the router's MAC address. The sending host has no knowledge of the MAC addresses of these other hosts outside its own IP subnet. As far as that sending host is concerned, the only MAC address for all those remote destinations is the MAC address of the default router!

Bert