I changed device(LTM 10050s to 10350v-fips) last weekend. When replacing, I changed the external_tr interface from untag to tag on the external VLAN. In the case of an interface tag, the drop count increases by one every 3-4 seconds. Changing it to untag doesn't happen.
The device connected to that interface is a cisco nexus device.
The i4600 equipment was also replaced and tagged under the same conditions, but there was no drop count.
Tagging VLAN is needed when you have multiple VLANs data and to differentiate between packets arriving from different VLANs by Tagging each packet to designated VLAN for easily difference, that mean more VLANs means Trunk port , have you configure trunk on both side LACP etc onfigs.
When you use only one VLAN there you can have it untagged and the switch on other side know the VLAN due to the one or native VALN, and hence tagging is not needed and the kind of port we access are then ACCESS ports not the TRUNK port.
Check if you need TRUNK port use TGGING for the ports or ACCESS port or one VLN then no TAGGING or UNTAGGED and proceed accordingly.
Also consider port bundle using port channels for increase in the bandwidth or through put and assign it to the trunk port.
Let me know for any more details