The warm spare automatically gets the same local VLAN and interface config. In an L3 CORE design, just have a transit VLAN that is a /29 or lower because you need at least 3 IP's for your switches in a stacking scenario and 1 IP for the uplink MX. Use access ports on the switches connecting to the MX'es. In an L2 CORE design, use trunks between the switches and the MX'es and use one of the configured VLAN on the MX as native VLAN on those trunks because you should not "drop untagged" traffic because STP BPDU's are sent untagged and I have seen four links in forwarding state due to this.
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