The auto MDI/MDIX feature detects if a port requires MDI or MDIX configuration on EX Series switches. On EX4300 Series switches, auto-MDI/MDIX is also disabled when auto-negotiation is disabled.

This article suggests a workaround and indicates the Junos OS release in which this behavior is changed.

When you turn off the auto-negotiation feature to provide a fixed speed/duplex setting on EX Series switches, auto-MDI/MDIX stops working. If you are using a straight cable and auto-MDI/MDIX is required (for example, for an MDIX-to-MDIX connection), the link will fail to come up.

On EX4300 Series switches, auto-MDI/MDIX requires that auto-negotiation must also be enabled.

As per PR1212093 , this behavior has been changed in Junos OS releases 14.1X53-D40 and later wherein auto-MDI/MDIX will work even though auto-negotiation is disabled.

Meanwhile, to work around this problem, use a crossover cable instead of a straight cable.

2019-03-27: Modified solution to include Junos OS release in which fix is available; made other minor non-technical changes