Seems running 19 code trains on EX 4300-MP with the 4x10G SFP+ uplink module connected LR / SR to Cisco switches
(3850, N7k9, 4500x) you have to set speed nonegotiate . If you do not you will see link lights on your Juniper gear but not on the Cisco side.. We thought we have bad fiber / spf but as soon as we swapped out 4300-mp with 3400 or 2300 the links came up.
Did a little tshoot / wild guessing and found you have to set speed nonegotiate on your Cisco side when connecting at 1gig.. 10gig works fine.. Hope it helps someone .
interface TenGigabitEthernet1/1/4
description uplink-2260-ex4300-10gig
switchport trunk native vlan 3967
switchport trunk allowed vlan 9,10,12,22,36
switchport mode trunk
switchport nonegotiate
speed nonegotiate
udld port disable
spanning-tree link-type point-to-point
ip dhcp snooping trust