99% of the time, the problem is fiber polarity — specifically, Transmit (Tx) talking to Transmit and Receive (Rx) talking to Receive instead of Tx ↔ Rx. Good news: it's incredibly easy to understand and fix once you know the “two-lane highway” rule. Recently some of switches I have has been shown faulty issue on Fiber port. I have seen this kind of issue on both '3750X series and 2960X series', and issue is following: - Swithes are 48- Ethernet ports PoE + 4 Fiber ports - 3750X switch = 3/4 ports are currently plugged with Cisco GBIC module. the problem coming when I try to connect IDF MS225 which is ground floor and C9300 MDF on second floor. Fiber is full-duplex, which means it always uses. Switch A is on the router end, devices connected to this switch get DHCP leases and can browse the internet without issue. 5gb equipment after they upgraded there Comcast connection. The information in this document is based on all Catalyst 9000 Series switches.
