Switching

Hi All

I'm curious about how switches identify frames as being part of a primary vlan AND secondary vlan (community in this case).
I have two switches set up in the below topology (ignore IP addressing labels).

vEX1 Config:

set vlans finance vlan-id 20
set vlans finance private-vlan community
set vlans sales vlan-id 10
set vlans sales private-vlan community
set vlans vlan-pri vlan-id 100
set vlans vlan-pri community-vlans sales
set vlans vlan-pri community-vlans finance

set interfaces ge-0/0/4 unit 0 family ethernet-switching vlan members sales

set interfaces ge-0/0/1 unit 0 family ethernet-switching interface-mode trunk
set interfaces ge-0/0/1 unit 0 family ethernet-switching inter-switch-link
set interfaces ge-0/0/1 unit 0 family ethernet-switching vlan members vlan-pri

vFGT Config:

set vlans finance vlan-id 20
set vlans finance private-vlan community
set vlans sales vlan-id 10
set vlans sales private-vlan community
set vlans vlan-pri vlan-id 100
set vlans vlan-pri community-vlans sales
set vlans vlan-pri community-vlans finance

set interfaces ge-0/0/2 unit 0 family ethernet-switching vlan members sales

set interfaces ge-0/0/1 unit 0 family ethernet-switching interface-mode trunk
set interfaces ge-0/0/1 unit 0 family ethernet-switching inter-switch-link
set interfaces ge-0/0/1 unit 0 family ethernet-switching vlan members vlan-pri

VPC6 (192.168.10.50/24) can ping VPC7(192.168.10.51/24)

I would have thought that doing a PCAP on ge-0/0/1 on either side whilst vPCs were pinging would show the primary vlan-ID but instead it shows the community vlan tag.

Even though only vlan-pri is setup on the trunk link.

My question is:

What exactly is happening here that I can't see in the PCAP but is obviously happening on the switch itself?

Cheers!