Physically the electrons must pass through both switch and router, because despite there being 2 subnets (one on each port of the router) they are all connected to the same switch.

So you are right. What is your colleague saying?

Edited to add: the routers job is to look at packets on port A and if any are destined for the other subnet deliver them to port B. If no traffic went via the router it wouldn't be doing anything at all!

[Edited by ajm - 11/6/2003 10:13:48 PM]

... furthermore, if you want to get him totally, look at it this way...

from a networking point of view it would make absolutley no difference to the client PC's if that one switch were actually 2 separate switches, one for each subnet. The routers two ports would be plugged into the two switchs and would be the only physical link between the two. Then ask him how the traffic gets from subnet A to B without going through the router!

[Edited by ajm - 11/6/2003 10:22:15 PM]

Lets say I have a Cisco Router connecting to a switch (say just a bog standard 3com) via ethernet and then 10 machines running on one subnet and 10 on a different subnet. Bearing in mind the routers ethernet has two Ip addresses assigned (one from each subnet).

If I want to communicate from subnet 1 to subnet 2 - does the traffic go via the switch and router or just the switch?

I would say via both switch and router as the router connects the subnets to each other. However a colleague disagrees and I have thought about this way too much so Im just going around in circles.

Cheers,
Simon.

The data goes through both switch and router. Else whats the point of subnetting lol.

Absolutely with a seperate basic router and switch with multiple subnets on the same VLAN - traffic between subnets goes via the router. There is no mechainsm for the end Hosts to send IP traffic directly to each other if their interface addresses are on differnt subnets.

Unless....

You activley configure it differently. High end Cisco Switches and Routers can be configured such that only the first packet goes via the router (and susequent traffic in that conversation is forwared direct from switch ingres port to switch egress port. (Multilayer Switching or MLS)

Deano