R1 needs a higher priority to make sure it's the active gateway so we need to reduce the priority on R2 and then we put the preempt command on R1.

CoS value 5 is commonly used for VOIP and CoS value 5 should be mapped to DSCP 46. DSCP
46 is defined as being for EF (Expedited Forwarding) traffic flows and is the value usually assigned to all interactive voice and video traffic. This is to keep the uniformity from end-to-end that DSCP EF (mostly for VOICE RTP) is mapped to COS 5.
Note:
+ CoS is a L2 marking contained within an 802.1q tag,. The values for CoS are 0 7
+ DSCP is a L3 marking and has values 0 63
+ The default DSCP-to-CoS mapping for CoS 5 is DSCP 40