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.
no-auto-negotiation
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:
Delete the ether-options stanza on ge-0/0/1.
Commit the configuration.
Set the ether-options properties again to 100/full/no-auto.
Now you can see that EX connects to Cisco with 100/full correctly, as shown below: