I use a single MX84 at many sites, connected to two 9K switches running VPC/HSRP for L3 etc. What I usually do is on CORE2, I'll set the port the MX connect to to have a lower spanning-tree cost, so core 2 is the one that blocks the port. spanning-tree vlan XXX cost 10 Never had any issues yet. Tested failover from both sides, unplugging cables, powering off one 9k etc. Fail-over is pretty quick and I prefer having some form of redundancy vs nothing. Just really with the MX line would start supporting LACP or something.
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