I was reading over some JNCIS-SP documentation on provider bridging and it had this to say regarding the TPID field for QinQ:
IEEE 802.1ad has reserved a TPID of 0x88a8 for the S-TAG however the JUNOS operating system default behavior is to set the TPID equal to 0x8100.
Wikipedia has the following listed for the notable protocols for these TPID values:
0x8100 VLAN-tagged frame (IEEE 802.1Q) & Shortest Path Bridging IEEE 802.1aq 0x88A8 Provider Bridging (IEEE 802.1ad) & Shortest Path Bridging IEEE 802.1aq
What is the functional difference between these two TPID values? Why would Juniper use a different default TPID than the IEEE reserved TPID for the S-TAG?