cascading access layer switches and prevent loop

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Oct 2nd, 2007
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i would like to inquire if it a common practice to cascaded switches below the access layer


A and B are connected to dist switch

| |

switch A ------- switch B

| /

switch C,D,E,F,G,H - i want them connected to A and B so i will have redundancy

i am thinking of letting the switch run the spanning tree automatically, however, i am hesitant as it might create a loop. Will the loopguard ensure i am loop free? and must i configure the spanning tree point-to-point

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Kevin Dorrell Tue, 10/02/2007 - 23:49
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As long as you have only the three lavels, and you are running Spanning Tree on all the switches, then it is reasonably safe.

You might want to put the Spanning Tree in rapid-pvst mode for faster convergence in the case of a link failure, and/or configure uplinkfast on switches C-H at your bottom layer. Also, make sure the roots of the Spanning trees are at the top.

If you take all those precaustions, there should be no need for any fancy configuration.

Kevin Dorrell


Jon Marshall Tue, 10/02/2007 - 23:52
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Hi Eduard

A few more details would help.

Do you have only one distribution switch because if you do that is your biggest single point of failure.

In general cascading access-layer switches is not recommended as this just increases the STP diameter.

Without knowing your topology it's difficult to comment more but a more traditional redundant design is to have 2 distribution switches and connect each access-layer switch to each distribtion switch.


edongskiu Wed, 10/03/2007 - 01:11
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hi Jon,

I got 2 cisco multilayer switch for the core layer, 4 3700 for distribution layer (all are interconnected) and 6 main access switches (paired), where each main access switch has 10 to 12 cascaded switch more. the edge switches are the ones i want to have 2 connections on the 2 access switches (paired in my case).

core 1 - core 2


distrbution switch (4)

| | | |

A1 A2 B1 B2 C1 C2 D1 D2

x x x

1,2 3,4 5,6 7,8

where i want 1 connected to A1 and A2, 2 connected to A1 and A2, .....



Jon Marshall Wed, 10/03/2007 - 01:14
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Hi Eduard

Okay you are within recommended STP diameter limits within this setup. As suggested by Kevin you should be okay with this configuration although i would always try to look to connect access-layer/edge switches direct into the distributioin switches where possible.

As Paul states, you want to keep your configuration/topology as simple as possible.




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