im usind ifAlias for tagging interface. we have about 2-6 physical interface for each logical interface. for example GGC1-1 contains 5 physical interface with ifAlias "GGC1-1"

today i calc total traffic for all GGC* with this method:

sum(last_foreach(/*/interface.ifHCInOctets[*]?[tag="interface:GGC1-1" or tag="interface:GGC1-2" or tag="interface:GGC1-3" or tag="interface:GGC2-1" .... and more and more]))

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with regex it will be mutch more simple:

sum(last_foreach(/*/interface.ifHCInOctets[*]?[tag="interface:~GGC\d+-\d+"]))