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[EX] The behavior of Gigabit Ethernet interfaces when no-auto-negotiation with 1000/full is configured



Article ID: KB22315 KB Last Updated: 05 Mar 2017Version: 3.0

This article describes the behavior of Gigabit Ethernet interfaces, when no-auto-negotiation with 1000/full is configured.


According to the IEEE 802.3 1000BASE-T standard, auto-negotiation on copper GE interfaces is mandatory. However, sometimes no-auto-negotiation with 1000/full mode could be mistakenly set to the GE copper interface.

EX will force the enabling of auto-negotiation, in spite of the no-auto-negotiation statement; this may lead to serious issues for the EX device.


If no-auto-negotiation and 1000/full is set on the copper GE interface by mistake, the configuration can be committed normally without any warning or error. It is difficult to actually notice such a configuration as an invalid one on the GE copper interface, as Junos OS always enables auto-negotiation when the speed is set to 1000m on a GE copper interface.

For example, In the following topology, the EX copper interface side is set as no-auto-negotiation with 1000m/Full. It can still work well, if Cisco is set as 1000m/Full with auto-negotiation enabled.

But if the connection on the Cisco side is changed to a FE port (with auto-negotiation enabled), the connection will not work on that interface and you will see a collision error. This is due to EX having an invalid configuration, such as 1000/full with no-auto-negotiation; subsequently auto-negotiation will keep on activating on that interface.

As auto-negotiation is never turned off, even in the EX configuration, the speed is changed from 1000m to 100m. So, auto-negotiation could fail and lead to EX working as 100m/half with the Cisco device, as shown below:

To correct such an invalid configuration, perform the following procedure:

  1. Delete the ether-options stanza on ge-0/0/1.

  2. Commit the configuration.

  3. Set the ether-options properties again to 100/full/no-auto.

  4. Commit the configuration.

Now you can see that EX connects to Cisco with 100/full correctly, as shown below:

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