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I'm planning for some upcoming changes in our network topology and I'm going to have a stack of 4 PowerConnect 3548P switches, to stack I need to use their GbE ports which is fine as they also have 2 x fibre Gb ports which I can use to connect to my main 'core' switch which also has only 2 x fibre ports.

Now from what I've read the ideal situation is when you have redundant paths to the switch stack, so I'm planning to connect 1 fibre cable to the master switch and the other one to switch nr 3 or 4.

On the main switch for the network should those 2 ports be on a LAG or not?

Any advice welcome.

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up vote 1 down vote accepted

It depends.

Just connecting two of your stacks' switches without configuring a LAG would result in a network loop which would be kept disconnected by Spanning Tree (i.e. one of your links would be constantly held in a "blocked" state). In the case of link or switch failure, the failover times for STP are approximately 50 seconds, RSTP would be considerably faster around 1 second.

Configuring a LAG would result in a trunk where both links would be held active and traffic would be distributed among them depending on the algorithm chosen. The failover / link failure notification time would depend on the implementation but typically is within the magnitude of 1 second.

If you do not need the additional bandwidth brought by Link Aggregation, it is mainly a question of which implementation you trust more to handle all of your possible failure cases gracefully. You might want to subsequently set up both configurations and test a couple of failure scenarios and see if the failovers work as expected and the times suit your needs. Some examples of what you could do:

  • switching off a stacked switch connected to the uplink
  • pulling one of the uplink cables
  • breaking (or over-bending) the glass fiber of one of the uplink cables (preferably two times - once for each direction's fiber)
  • completely separating out a single switch connected to one of the uplinks out of your stack by disconnecting both stack links
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Very detailed answer, more than expected. Thank you! – hyp Mar 8 '13 at 15:03

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