Member Avatar for ibthevivin

Quick question. I'm studying for my CCNA and my supervisor gave me this challenge question.

You're given 5x Cisco 3560's and 5x Cisco 3750's which are to be installed side-by-side on a rack. Which model would you choose to avoid issues with STP?

I'm currently skimming over this.

Thanks!!

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Short answer: pick the Catalyst 3750s and put them in a StackWise stack. A 3750 stack behaves as a single logical switch (one control plane) so STP runs once for the whole stack rather than once per physical chassis — that dramatically reduces the risk of STP “domino” reconvergence when a link or box flaps. (cisco.com)

Why: the 3750 family was designed for StackWise (up to nine members) and presents the stack as a single switching unit to Layers 2/3, which is exactly what you want when you’re trying to minimize STP complexity. The 3560 family does not support that same StackWise switch-stacking behavior, so five separate 3560s will each be independent STP participants (more BPDUs, more possible root/port changes). ’s hint about the STP “domino effect” is the key point here. (cisco.com)

Practical notes (what to do if you choose 3750s):

  • Wire the StackWise cables per the hardware guide so the stack forms a single logical unit.
  • Ensure all members run the same IOS version.
  • Pick the member you want as stack master by setting its stack priority (example below).
  • Lock the STP root on the stack (so the stack is the root for your access VLANs) with the STP convenience command below. (cisco.com)
configure terminal
 switch 1 priority 15
 spanning-tree vlan 1 root primary
end

If you must use the 3560s, keep them at the access layer (use L3 uplinks at distribution), aggregate links with EtherChannel where possible, and enable protection features like Root Guard / BPDU Guard on access ports to limit accidental topology changes. Those measures won’t give you a single-switch control plane, but they will reduce STP churn. (cisco.com)

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Member Avatar for BigPaw

I'm not going to give you the answer, but I will give you a clue as to narrowing down the amount of skimming you will need to do. Consider the issues of STP's and their domino effect, and then consider the capabilities of the two models. The answer can be deduced from that PDF, so he/she has gone quite easy on you. :-)

Edited by BigPaw
Member Avatar for ibthevivin

hahaha! Alright. I'll give you an update on where I've gone. Thanks.

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