Strange Network Issues at One Building

[CCSTyler]

So I work for a public school district, and we have several buildings all linked up in a Meraki Network. All our secondary buildings are linked to the main building core VIA fibre cable. All settings are pretty much the same between the buildings as well. However, recently we noticed 1 building is having trouble. Specific MISD websites aren't loading correctly or taking forever to load when you access them from that building. At the same time other websites are working fine. Furthermore we noticed trying to access one of our servers from that location either runs slowly, or randomly disconnects. But accessing other servers work fine. 

Our immediate assumption was that there was a connection issue with the fibre, but the problems don't affect everything, just some websites or some servers, not all. 

At this point we don't even know where to look to diagnose the problem, anyone else have any suggestions?

[cmr]

@CCSTyler do you have a stack with L3 routing on at that site?  What switches, a router, or???

[CCSTyler]

Ah should have clarified from our MDF we have a MS425-32 Core with a fiber module that links to a MS120-48FP switch at an elementary school. That switch supports our AP infrastructure, which consists of MR42s and a few MR52s. The switch also feeds a  MS220-24 switch which manages all our wired devices. 

[cmr]

Thanks @CCSTyler is the broadcast domain the 220+120 and just one VLAN interface on the 425s?  Are the failures consistent and if so can you set up a second port on the 425s in the same VLAN and test?

[CCSTyler]

We tried a different port on the 425, and then tested with a different fiber module. Same issue. We did however run a speedtest and noticed the connection was fairly spotty, so we are currently thinking the specific websites we are accessing may not be dealing with intermittent packet loss very well. That's why some sites are fine but these aren't. Strangely even though the download speed was intermittent the upload was consistently good. I think our next step may be to test all our fibre lines to see if one is defective. 

[PhilipDAth]

I would open Chrome developer tools (CTRL-SHIFT-I), load the page, and see what specify is having an issue.  Is it DNS, is it loading a specific component from a third-party CDN - what.  That will give you a strong direction to pursue.

Because only certain web sites are affected; I would compare things like the returned DNS value for a working site against a non-working site.  Perhaps they are landing on different CDNs.

Everything else below is not probable.

Have you configured the STP root bridge to be your core switch?

https://documentation.meraki.com/MS/Port_and_VLAN_Configuration/Configuring_Spanning_Tree_on_Meraki_...  

You could verify the fibre is working ok.  I would use iperf3 on a machine on both sides of the fibre (could be a server on one end and a workstation on the other) and measure the throughput on the fibre to make sure it is as expected.

https://iperf.fr/iperf-download.php