Currently with all of the devices set to the same priority the switches will elect the root switch based off MAC addresses. This creates a scenario where the root switch could be one of your SG300 switches, and that is not ideal. I would use the show commands to gather information and map out the current root switch, which ports are designated, root, and discarding. Knowing this information can help explain why you are seeing communication on some links but not others. The spanning tree changes can cause some disruption in the network as topology changes occur. Be sure to plan out what changes are going to be made and do the changes during planned non peak hours.
Hello Daniel, I think you are in a whirl of your business. If you are Okay, can you help me about network setting? It's a little bit long story.  At first, I'll show you my topology. [IMAGE]  As you can see the topology, there are 3 switches ; Dell 8024F, Dell 7024P, Cisco SG300. Actually, I originally set up the machine to L3 mode except for Cisco SG300 devices (L2). So I set these switches like followings. 1. Routing protocol at 7024, 8024 : OSPF (L3) 2. Dualization between Dell 7024 : VRRP (L3) 3. Vlan 1 ip setting at SG300 : L2  That is, above 7024 switches, L3 mode, other side is L2 mode. Did you get it what is going on..?  If you understand until this, I'll mention continuously.  After setting, everything is Okay. That is, the whole devices communicated each other except for that I indicated "My company's Display Extender" which is used for TCP, UDP in video at the below of left side. When I used "TCP" at Display Extender in video, it was going alright. But, I used "UDP", It was not going alright. To sum up this, like below picture. [IMAGE] Do you think this situation might be characteristic for own our device? For that reason, I changed from L3 to L2. In this point, only thing that I set was vlan IP assignment. Until this also, it was going alright. A few days later, some issues occurred.  1. When I added 8ea x 7024P, 32 x SG300 below 8024F, the whole network connected 7024P was disconnected suddenly. => We set SG300 only VLAN assignment.. so I supposed it must be going well.. Perhaps would you think the traffic is cause the disconnection due to lots of devices?  2. When I changed "Display Extender" device to another Extender, it was not communicated properly even though another Extender's IP was same as previous Extender. => In theory, It was out of  common-sense. I don't understand what is this..  ★3. Related to 8024F, I'll let you know what is the problem. First, I pick out the ①cable  , communication (OK) Second, I put the cable original status. Third, I pick out the ②cable, communication (Failed) [IMAGE]  How can this situation happen?;; In the theory, I don't understand, maybe you too.  I really appreciate all you efforts. Please contact me.. Thank you expert.
To Denial,,,