i'm new here...
If I have an old PC with a 100Mbps Ethernet and I install OpenWrt, and I want to connect it to a 1-gigabit managed switch, will I get 1-gigabit speed on each port of the switch?
Between all the other gigabit ports of the switch, yes.
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so it Doesn't affect my internet speed and communication between PCs connected to the switch?
Unless the traffic in some way flows through it, no.
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What is the purpose of the PC with OpenWrt on it? Is it being used as a router? Or something else?
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i will use old openWrt pc as a router, but only with 1 lan from that computer connect to my 5 port gigabit switch as wan and lan to my pc...
To clarify -- you are trying to make this a router with only a single physical network port, correct? This would require a managed switch -- do you have one [EDIT - I see that you said you have one in the title]? And with a 100Mbps ethernt port, this would limit the router to 100Mbps maximum speeds (i.e. to the internet).
EDIT: adding detail..
you'll connect your computer to one port of the managed switch, and the actual internet connection will connect to another port on the same managed switch. You will need to assign a VLAN for the WAN on the switch and there will be 2 ports that have membership in that VLAN -- the ISP connection (probably untagged, but depends on the ISP details) and the computer (ideally tagged). The port that connects to the comptuer will also need a VLAN for the lan (ideally tagged). On OpenWrt, you'll set your network interfaces to match the tags from the switch.
Keep in mind that all of your internet traffic (and if you use other VLANs, inter-vlan routing, too) will flow through this 100Mbps device, so the max speeds you will ever see will be 100Mbps if routing is involved.
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Well, maybe... We had some old 24-port 10/100/1G switches from around 2006-8 that would throttle ports in groups (4-ports/group iirc?) to the slowest link in the group. I suspect it was a hardware thing, one set of registers per port group or something like that. It's been long enough I don't remember the vendor, but I do remember the behavior, as we still had some 10 or 100 Mbps voip phones and had to make sure they weren't in the same groups as the workstations.
Hopefully this is not an issue with new gear these days, but if you pick up some old stuff, it may come back and bite you.
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While not an answer to your question, also consider the power consumption - x86 devices with 100 MBit/s ethernet cards tend to be at least almost 15 years old by now, unless we're talking about a notebook, we'd probably be looking at >90-115 watts idle there. Even if you are in a location with cheap electricity, this would be a lot (compared to 5-20 watts for modern devices, including well-selected modern x86_64) - new hardware can easily pay for itself in under a year under those circumstances.
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Perhaps this is the topology that I mean
so it still only get 100M speed right???
thx for all the answer..I greatly appreciate it.
Yes, you will be limited to 100Mbps. Why do you want to use that machine as a router? You have two APs - they may be capable of better throughput at lower power consumption. What models are they?
If you need to spend that money to buy managed switch and pay for power bill of your old PC as router running 24x7, why don't you just pay for a proper router? Since you are fulfilled with 100M, I would say any router capable to run latest OpenWrt release on market can fit your needs without breaking the bank.