After numerous headache with our Cisco 2921 routers and the RPS, I decided it was time to bring a 6503 back from the dead to handle the edge routing in our office. One problem though, I couldn’t get our friggin’ AT&T fiber connection moved to the 6503, even reusing the same cable and SFP. Port Gi1/1 on the SUP720-3BXL was configured with the exact same information as the 2921, yet the link would not come up.
I swapped the GLC-LH-SM for a SFP-GE-L, which is the fancier version that supports DOM (Digital Optical Monitoring). This showed the signal was fine, indicating the SFP and cable were not the issue:
Switch#sh int gi1/1 transceiver Gi1/1 29.7 3.29 6.1 -7.1 -6.4
After several days of head scratching, a Reddit user saved my butt and pointed me to the problem. For whatever reason, disabling speed negotiation fixes everything. This makes no sense since a fiber SFP would only support 1000/full, but I gave it a shot and disabled speed negotiation entirely.
interface GigabitEthernet1/1 description AT&T 500Mb Internet bandwidth 500000 ip address 126.96.36.199 255.255.255.252 no ip redirects load-interval 30 speed nonegotiate no lldp transmit no cdp enable
Upon adding that, link light came on and interface was up/up. Yet another entry for the Cisco gotcha hall of fame.