How many network switches can I daisy chain together [duplicate]

We have a number of custom computer systems that need to be networked and temporarily installed around a football stadium - each computer has it's own network switch. These computers all need to be networked to a central computer. Up to this point, we've been daisy chaining the switches so as not to exceed cable length limits and putting the server in the middle of the chain with 5 computers/switches going off on one chain, and 5 going off in another chain and everything has worked great. I checked network utilization on the server when looking at the network traffic from one computer, the utilization was about 0.13% of the gigabit connection, and all of the switches at each computer located throughout the stadium are gig switches.

We're running into some situations where we need to increase the number of computers/switches and possibly move the server to the end of the chain, meaning the server could be at the end a chain of 20 switches. Home-runs are not an option due to exceeding gig network cable lengths (300ft) and fiber would be much too costly.

So, I know that it's not recommended to daisy chain network switches, but I haven't read anywhere hard and fast that says you can't do it. Further, there is no other traffic on this network other then those computers which send small amounts of data over the network to the server.

Besides not being recommended for a standard office network (which this is not), will we run into any major issues? When daisy chaining multiple switches, is it just a matter of excessive traffic on the uplinks closest to the server, or are there other switch routing/timing/processor issues that we would run into?

Thanks! Mike

Solution 1:

There are no hard limit on daisy-chaining switches. I had a layer 2 network once where a PC at one end of the network, reaching a server at the other end could easily go via about a dozen switches. But there is a practical limit depending on hardware and your network needs.

How many can you daisy chain depends on what's happening on your network and the quality of your switches. A "gigabit switch" doesn't mean a thing really. How much packets can that switch process? How much bandwidth can the switch forward?

Also, what's the network traffic? Are all PCs only sending/receiving traffic with the servers or is there PC to PC traffic?

Assuming the traffic is only to the server, with what you posted, I wouldn't be afraid, but I would make sure to use good quality switches.

One thing to keep in mind is what I personally call the "inverted funnel". If you have PCs mostly sending traffic to a specific point, you can, by daisy chaining arrive to the situation where you have something like this:

PC ---|
PC ---|
PC --- switch1 ---- switch2 --- switch2 --- switch3 --- SERVER
PC ---|               |            |
PC ---|               |            |
          PC ---------|     PC ----|
          PC ---------|     PC ----|
          PC ---------|     PC ----|
          PC ---------|     PC ----|

So in the example above, if every PC is downloading 10M from the server, how much traffic is going through switch3? 13 x 10M = 130M. So if all the links above were 100M links, then you would choke. The above is what I call a "funnel design".

The inverted funnel design would require that the bandwidth between switch1 and switch2 be enough to carry the expected load from all the nodes connected to switch1. Then the link to switch3 from switch2 would have to be enough to carry the load of switch2 and switch1, etc. So the link between the switches would have to be greater than the link from the nodes to switch. Example: 24x100M ports switch has 2 x 1G connection to next switch. That being obviously based on expected bandwidth usage, not the port speed.

If you have gigabit ports but all nodes are using only a few Kbps, then it may take a bit to saturate - bandwidth-wise.

You have to do similar math on packet quantity.

Solution 2:

There isn't really any limits about connecting switches, but you should be aware that there is limits of the spanning tree protocol is limited to 7 hops.

Twenty switches between two systems seems unusually large, you probably should consider breaking things up into VLANs at that point. You can do this using the layer 3 features of a switch even.

Refs:

• http://slaptijack.com/networking/max-spanning-tree-stp-diameter/
• https://supportforums.cisco.com/discussion/10057486/8021d-spanning-tree-7-hops-limitation
• http://www.computerworld.com/article/2581420/disaster-recovery/all-systems-down.html