Using 1 Gbps Fiber SFPs on a Cisco 6500 supervisor

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
 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.


48 Port Copper Modules for 6500 Series

WS-X6148 & WS-X6548

  • No Jumbo Frame Support
  • 1MB Buffer per 8 ports (average 128KB per port)
  • Not supported with newest Supervisors (VS-S720-10G & VS-SUP2T-10G)

WS-X6148A & WS-X6748

  • Jumbo Frame Support
  • 2.67MB or 1.3MB Buffer per port
  • Supported across all Supervisors

In short, the 6148 and 6548s should only be used in low-traffic environments, with SUP32 and SUP720 (non VSS) Supervisor cards.  The 6148A and 6748 are for high-traffic environments, such as server farms