My understanding at the em0 control ports use internal only addressing and shouldn't be part of any routing instance. I've read some interesting articles on this in the past, but nothing worth sharing. Perhaps someone else may have something to add or can share a link on how this addressing is assigned. I hope this helps.
John Judge JNCIS-SEC, JNCIS-ENT,
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EM0 are internal interfaces that connect between the Routing Engine (RE) and the Control Board (CB). There is no correlation between EM0 to any physical interfaces.
The em0 corresponds to control link between clustered chassis. As for the 129.X addressing, it is used for intrachassis communication, and these addresses will be part of default private instance, which will not be reachable from global routing table.
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