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New SG300-52P brings down network

We have a network of 6 switches and are adding a 7th.  5 of the existing 6 are SG300 units and 1 is an old L3 Catalyst which is currently the gateway.  We tried to add another SG300-52P today but whenever we plug it in, the rest of the switches stop communicating with each other.  (The #'s are our way of identifying the switches).  The intent is to add this new switch for capacity and then eventually migrate away from the Catalyst.  We use VLAN2 for voice and VLAN10 for data.  This is how they all currently link together:


Catalyst_#3 <--Ethernet--> SG300_52P_#4 <--Fiber Link via FMC--> SG300_26P_#7 <--Fiber Link--> SG300_26P_#8


|-<--Ethernet--> SG300_52P_#5 <--Fiber Link--> SG300_26P_#9


The new switch an SG300_52P_#6 plugging into SG300_52P_#5.  Ports on each switch are configured as 2T, 10T, 4095P.  Everything is running fine inside of the network but the moment we plug the new #6 switch into the existing #5 switch it all just kind of falls apart and packets start dropping.  Once we unplug it comes back after about 10 seconds.


More info:

1.  All switches are configured as Layer3 but only the Catalyst has any routes configured.  The others are only configured as such in case the Catalyst goes down and we need to do some configuring on one of them to get the network back up.  Since changing them from L2 to L3 wipes the config it seemed like a good idea.

2.  I've read that the best way to interconnect the switches is to tag all the VLANs and leave VLAN1 excluded.  Some of our switches are interconnected with that config.  Some are interconnected with the VLANs tagged except VLAN1 is untagged.  Not sure why we did that. 

3. We haven't configured any other protocols such as STP.

4.  We have verified that the IP assigned to the new switch works just fine.


Any help would be appreciated.  Just when I think I've got the whole VLAN thing figured out something like this pops up and makes me realize there must still be some things I don't quite get.

VIP Advisor

Hi @StewJo ,


To discard, can you check the status of VTP?

Maybe you can try leaving the new switch in transparent mode (vtp mode transparent).



It looks like the SG line doesn't support VTP but rather GRVP.  It's disabled on the switches.

I wasn't able to figure out what happened, but I brought the new switch and firewall online and moved the existing equipment over to it.  As long as the Catalyst wasn't connected into the network there weren't any problems.