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Connect 2 Netgear switches of different models via uplink does not work

One netgear switch of the company i work for was RMA’d and we got a replacement of the ” M5300″ series. i connected the switch so our backbone switch via SFP+ 10GBit uplink but the two switches did not want to communicate. it was like the cable was broken or sth, but the port was displayed as “connected” and “forwarding”, after some hours of trial and error i gave up and we contacted netgear support, they said the VLAN configuration must be wrong (but i was sure it wasnt) so they decided to look together via teamviwer and i told the support guy what i was expecting and what the results are. he saw the config UI of the new switch and said “seems right”. then i connected to the “old” switch to show hin the VLAN configuration and he saw the model number and the old web UI. he said we need to connect back to the other switch (the new one, he might have a solution…)

we disabled STP on the uplink port and tadaaaa, it works.

he told me that when connecting old models with newer ones this is a known issue.

TL;DR

so, to fix an uplink between two different models of netgear switches go to the following menu and disable “Port Mode” for the uplink port (in this case 49) on the newer switch: