I need help to finish the ring on a Ring Topology please

[aphudson112211]

Hi all. I'm new to networking and inherited a Ring topology network that was never connected at one box to complete the ring. It is made up of an LC Fiber ring of all Cisco 3750X switches all set up on the same VLAN. I am in no position to change the architecture. Will someone please tell me and/or direct me to something that would indicate what the configurations on these switches should be so that when I connect the fiber to the last switch, I don't take down my whole network. Thanks in advance. 

[Joseph W. Doherty]

If your switches are running PVST, the default, closing the ring shouldn't cause an issue unless it's a very large ring (if STP's diameter is exceeded).

If you add a new switch, unless you have an assigned root, a root change will pause the network, which can take some time if not using a rapid STP variant.

[aphudson112211]

Thanks for the response @Joseph W. Doherty. I currently have 15 switches but we are going to trim this down since its unnecessary to have that many but we will still have more than 7. Further research has indicated that I may need to study up on REP. Thanks again.

[Joseph W. Doherty · Accepted solution]

I was going to suggest looking into REP, but didn't recall whether the 3750-X supports it.  I recall it wasn't in the past, but possibly a later IOS version might.  (Just found REP is provided in the later 3750-X IOSs.)

If you have 15 switches in a chain, if you're running PVST, you've probably already busted the 802.1D diameter recommendation, but that's also assuming default settings.  From various articles, on Cisco platforms, you can usually push PVST parameters to support a diameter of 17 to 19 (depending on pushing all the values).

However, a much better choice for STP would be rapid-PVST or MST, both, I believe, supported on the 3750X.  The 802.1w standard much better supports a larger diameter, and from what I've read, your 15 switches wouldn't be a problem for it.  Lots of other good reasons to use a rapid STP too.

Some Cisco TechNotes on PVST and rapid-PVST.

That said, if you IOS does support REP, possible that would be the better choice.

[Georg Pauwen]

Hello,

in addition to what @Joseph W. Doherty says, it might help to post a schematic drawing of your topology, showing how your switches are connected. A 'ring' might not be the best way to connect these switches...