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If you connect a trunk port instead of an access port from your managed switch; does the bandwidth not get cut in half when the vlan is accessed on the managed switch's access port for that vlan?

leeand00
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    I'm sorry... what? – joeqwerty Apr 28 '20 at 19:10
  • @joeqwerty A port on a router can be configured as either a trunk port (tagged port?) or an access port. A trunked/tagged port is for connecting another router and sending along with it traffic from specified vlans to a managed switch. Then on the managed switch you can configure the untagged ports to provide service to specific vlans. – leeand00 Apr 28 '20 at 19:16
  • Yes, I know what trunk and access ports are. I'm having a hard time understanding what you're asking. – joeqwerty Apr 28 '20 at 19:17
  • @joeqwerty Just plugging in a plain old unmanaged switch halves the bandwidth. So I am asking if using another router and trunk ports on a router and a managed switch gives you more bandwidth than the half you get with the unmanaged switch. – leeand00 Apr 28 '20 at 19:22
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    Granted I don't do networking everyday, but how and why does the bandwidth get halved? I've never heard of such a thing. A trunk port carries traffic and VLAN tags. An access port carries traffic but no VLAN tags. Why does this have anything to do with the bandwidth of said ports? – joeqwerty Apr 28 '20 at 19:41
  • @joeqwerty I don't know; that's why I'm asking. Every IT guy I've ever talked to has said that for every switch you hook up from your wall outlet the bandwidth decreases by half; which is the way I used to do it, until I found out you can do a trunk port between the routers to send additional vlans and such and I wondered if that fixed the bandwidth issue in addition to allowing you to assign different vlans to different ports on the managed switch. – leeand00 Apr 28 '20 at 19:47
  • I'm not sure what they meant, but I've never heard that. I don't have an answer. Hopefully someone else will chime in. – joeqwerty Apr 28 '20 at 19:51
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    I agree with joeqwerty; that's not how networking works. You could have 10 switches between two machines and you could still get the full bandwidth (of the slowest link) if there's no other traffic, no matter how many VLANs you have. – Esa Jokinen Apr 28 '20 at 21:03
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    `Every IT guy I've ever talked to has said that for every switch you hook up from your wall outlet the bandwidth decreases by half` - find better IT guys. Switches aren't in-series resistors. – mfinni Apr 29 '20 at 16:04

1 Answers1

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The short answer is No, it doesn't.

Let's be clear what we're talking about. When people use the term "bandwidth," they can mean several related but separate things:

The port speed is determined by the physical hardware. A gigabit Ethernet port means just that: a bit of information is transmitted every 1 nanosecond, or 1 Gbps (I know I'm simplifying things a bit here, but I'm trying to illustrate a point).

Throughput is the amount of data transmitted/time. It will be something less than the port speed. If you send 1000 bits every second, your throughput is 1 Kbps. Note that the bits are still transmitted out the port at a rate of 1 Gbps. So data will be transmitted in bursts.

Utilization is throughput / port speed. If you're sending 500 Mbps of data out the port, your utilization on that port is 50%.

People sometimes use the term bandwidth to mean any of these things.

Now, if you have a 11 port gigabit switch (10 user ports and one uplink port connected to your router), each user port can potentially transmit 1 Gbps. However, since all the user ports share the same uplink port, they have to "share" the maximum throughput of the uplink port. So if two users are transmitting at the same time, they each will get 500 Mbps of throughput. But if there's only one user transmitting, she will get the full 1 Gbps throughput.

Whether a port is an access port or trunk doesn't change any of these parameters. But adding another VLAN to the port could mean adding more users who are potentially sending data on that port. In that sense each user has to share the available bandwidth with more users. It's just like sharing a pizza: more people, smaller portions.

If you connect a second switch to the first, you increase the number of potential users, so more users are sharing the uplink bandwidth -- everyone gets a smaller piece of pizza. But again, if only one user is sending data, she can use all of the available bandwidth, so her maximum throughput is 1 Gbps, regardless of how many switches are connected.

Ron Trunk
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