Can I use 2 ethernet cables to connect two switches for higher throughput?

Posted on 2009-05-01
Last Modified: 2013-11-09
   Does is make sense to use two ethernet cables to connect two swithes together for higher throughput? Both switches and cables are Gigabit. If the throughput between the switches is 1GB if there is one cable linking the swithces can I use two cables for double the thoughput?
My goal is to give the workstations faster connections to the servers.
Or is that just crazy and my network will slow down.


Question by:LangtechEE
    LVL 3

    Expert Comment

    I don't think it would cause your network to slow down, however It wouldn't increase the through put of that segmant because the switch would alternate between the two connections (unless you set up static routes). however never both at the same time. I belive....

    Author Comment

    What I'm thinking is that with one cable between the switched all the workstations connected to a 5424 are sharing a  1GB cable to the 6224 and the servers. If there are two cables than the same workstations would now be split up over 2 1GB connections.
    LVL 2

    Accepted Solution

    You can use Link Aggregation on the switches, basicically combining the two 1GB connections into one 2GB connection, or four 1Gb connections into one 4Gb connection.

    Here is a basic writeup if you are familiar with configuring Dell switches:
    LVL 23

    Expert Comment

    Not only can you use link aggregation, you _have_ to use link aggregation if you want to connect two cables between a pair of switches instead of just one and get a throughput improvement.

    You need to manually configure this setting on both dell switches, and place the two ports into an aggregated link  (or "port-channel") on both sides  before you plug in that second cable  to either.

    Without aggregation, if you just plug in two wires, one of two things will happen, either:

    (1) Spanning tree protocol (STP) or other loop-avoidance is running on both switches.  In this case you get lucky, one of the links is blocked so there are no serious problems.    However, you don't have any speed improvement.

    *Setting up blocked links is occassionally done intentionally, however,  STP or EAPS can serve as a method of having "backup"  or failover connections, in these scenarios each of the 3 switches might have a cable running to each of the other 2, but one of the links will be blocked by the protocol.  If the main link fails, the protocol stops blocking the reundant link,  and traffic flows over the backup link.

    (2) or 2...  if no loop-avoidance protocol is on your switches, plugging two cables in like that creates a loop, which would take down your entire LAN after 5-10 minutes, depending on how much broadcast chatter  (all 3 switches would be so hopelessly indudated with broadcast storm traffic that no computers could actually use the network).

    The danger of (2)  is why you should make 100% sure you configure aggregation on both ports before plugging two cables into exactly the right ports, AND take the time to check what loop-avoidance protocols are running on your switches before bringing up an aggregation scenario,  i.e. is STP enabled, or are the ports on your switches just set to "fast forwarding"?   (If STP is enabled, there should be a 1-minute delay between the time any of your network port comes up and the time they move to "forwarding" state and start allowing traffic to be sent)

    By the way, aggregation doesn't improve _individual_ TCP connections, in general all traffic between the same pair of hosts will utilize the same link,  but it can still improves the overall throughput of your network a good bit.

    It reduces congestion caused by multiple users.   (i.e. if 50 users are transferring data, you can expect  a percentage of them will be divided evenly over each link)

    If many users are very frequently transferring data with someone on another switch,  you might even consider using 3  links instead of 2,  for even further balancing of the load between switches.

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