Hi The problem was the polarity of one of the OM3 fiber patch panels between the two areas. Changing the polarity of the fiber cable patched to that panel fixed the problem. I'm glad it was something trivial like that and not something subtle. Hope the above helps someone else. Thank you for your time and suggestions.
Hello everyone I've been trying to resolve this problem the whole last week and today I just don't know what else I can try. I'm hoping someone can help. I have a S5296F (switch1) - firmware: 10.5.3.6, which I am trying to connect with a N1500 (switch2) - firmware 6.1.7.21, using DELL 10G transceivers with OM4 cables. I have established that there is link between the locations using the Fiberlert pen on both ends. The pen will beep, when there is light. So the physical link is there but the TenGig ports I'm using do not come up. They are constantly down. I'm using port channels. interface port-channel80no shutdownswitchport mode trunkswitchport access vlan 1switchport trunk allowed vlan (various VLANs)vlt-port-channel 80 interface ethernet1/1/80:1 (speed manually set to 10G)no shutdownchannel-group 80 mode activeno switchportmtu 9216flowcontrol receive on --------------------------------------------- ip dhcp snooping trustip arp inspection trustip arp inspection limit noneswitchport mode trunk channel-group 1 mode activeip dhcp snooping trustip arp inspection trustip arp inspection limit nonespanning-tree portfastswitchport mode trunk Both transceivers are DELL SFP+ 10GBASE-SR What I have tried so far: -  checked that both switches can see the transceivers and they both come up as DELL EMC qualified: YES - tried different transceivers - checked logs but all I can see is messages about the transceivers being removed and inserted back in. Is there anything obvious I'm doing wrong? Please let me know. What else could I try? Thank you in advance.        
10G link between switches does not work.