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[SRX] Auto-MDI/MDI-X is disabled when auto-negotiation is disabled

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Summary:

When SRX300 series or SRX550HM and a PC are directly connected with a straight cable and auto-negotiation is off, the link will not go up. This article explains the issue and offers two solutions.

Symptoms:

When SRX300 series or SRX550HM and PC are connected directly with a straight cable and auto-negotiation is off, the link will not go up.

Cause:

For SRX300 series and SRX550HM, when auto-negotiation is disabled, auto-MDI/MDI-X will be also disabled and port will be MDI port. If the device (for example, a PC) which is connected to SRX port is also in MDI mode, the link will not go up by straight cable.

Solution:
We have two options to solve this issue.

1. Use a crossover cable instead of a straight cable.

2. On Junos OS 15.1X49-D80 and later releases, a new command was added to forcibly enable the auto-MDI/MDI-X function.
   #set interfaces ge-0/0/0 ether-options mdi-mode force
   
    root# set interfaces ge-0/0/0 ether-options mdi-mode ?
    Possible completions:
      auto                 Automatic mdi/mdi-x mode
      force                Enable auto-MDIX always

     When the  force option is configured, auto-MDI/MDI-X will be enabled even if auto-negotiation is disabled.

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