Catalyst 3650 needs 'speed nonegotiate' for GLC-LH-SMD link to Arista (bug CSCvc47165)
4
The poster spent two days trying to bring up a single-mode fiber link between an older Arista switch and a Cisco Catalyst 3650 (IOS-XE 3.7.5) with Cisco GLC-LH-SMD (SFP-LH-SMD) optics; the path runs through five patch cords in the same building and had been tested by the cabling contractor. At first one switch link-flapped and went err-disabled, later no link came up at all even though 'show interface transceiver' showed good light on both sides (TX/RX were swapped as a test). An old Catalyst 3560 with the same SFP linked fine to both the Arista and the 3650, but a factory-reset 3650 configured as a trunk would not link even directly to a Nexus 5K, and GLC-T copper SFPs between the 5K and the 3560 also failed.
- Hardware: Cisco Catalyst 3650, Cisco Catalyst 3560, Cisco Nexus 5000, Arista switch (older model, unspecified)
- Transceivers: GLC-LH-SMD (SFP-LH-SMD), GLC-T
- Form factor: SFP
Symptoms / messages:
link flap followed by err-disabled
no link although show interface transceiver shows good optical power
1G LX/LH SFP fails to link between Catalyst 3650 and Arista
Comments 2
The poster's own fix: configure 'speed nonegotiate' on the 3650 interface holding the GLC-LH-SMD; forcing a static speed only on the Arista side had not helped. All links came up afterwards.
Cisco bug CSCvc47165 describes this behaviour; its documented workaround is exactly 'speed nonegotiate', and the fix is expected in IOS-XE 16.x, which the poster did not adopt for feature reasons.