Aggregate Switch

Unanswered Question
Feb 3rd, 2009

Can you tell me the benefit of having a switch aggregate the connections of four other switches. What I mean is, if I have four switches, with a trunk line connecting one of the switches to my local LAN. Then I daisy chain those switches via Gbit trunk lines, what would be the purpose or benefit to having a aggregate switch? Would it just be one more failure point and more overhead? Calling it a aggregate switch is the same as calling it a distribution switch, correct?

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Joseph W. Doherty Tue, 02/03/2009 - 12:38

If you "daisy chain" switches, traffic from one end of the chain to the other has to transverse all the in between switches. If three switches each connected to a fourth, switches on the end of the daisy chain would have one less hop.

Also, when you daisy chain switches, you increase the load on the connecting links. With an aggregate switch, assuming its fabric supports it, you should have more effective bandwidth between switches.

You're correct, if the central switch dies, all your leaf switches lose connections to each other, but lost of a non-end switch in a daisy chain is going to break the chain too.

Elly Bornstein Tue, 02/03/2009 - 12:41

Yup, a distribution switch is just a fancy way to say a switch that is aggregating all your other switches with users attached that might be spread out over different floors/buildings/etc.

We do have topology/protocol recommendations for ring topologies as well. Rings provide redundancy and would require two points of failure before the network is segmented. Whereas the linear topology you are talking about just needs one link to go down and all downstream users are affected.

Remember as you daisy chain switches it causes spanning tree to take longer to converge since the diameter of your network is larger. core/distribution/access is designed to have smaller network diameters which allow spanning tree to converge quicker.

glen.grant Tue, 02/03/2009 - 16:26

Its also a pain in the you know what troubleshooting a daisy chained install and trying to find where people are located in a chain .

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