We are only inspecting one VLAN at this stage - the outer VLAN ID. Junos seems to default to 0x8100 so by not specifying a TPID we are looking for 0x8100 tags. My best understanding is that based on this the Juniper is looking for the 802.1Q standard. Thus:
"The Canonical Format Indicator (CFI) bit indicates whether the following 12 bits of VLAN identifier conform to Ethernet or not. For Ethernet frames, this bit is always set to 0. (The other possible value, CFI=1, is used for Token Ring LANs, and tagged frames should never be bridged between an Ethernet and Token Ring LAN regardless of the VLAN tag or MAC address.)"
From my reading it would also seem that when having the CFI bit set to be 1, if the frame was even accepted then the MAC address would be read in reverse.
Another interesting comment from the O'Reilly Juniper MX book is:
"Canonical Format Indicator (CFI)
This is a one-bit field that specifies which direction to read the MAC addresses. A
value of 1 indicates a noncanonical format, whereas a value of 0 indicates a canonical
format. Ethernet and IEEE 802.3 always use a canonical format and the least
significant bits first, whereas Token Ring is the opposite and sends the most significant
bit first. This is really just an outdated relic and the CFI will always have
a value of 0 on any modern Ethernet network."
I should have a good test case set-up soon and I should have a better understanding of what is going on within our MXs.
If anyone else has a better understanding of what might happen on a Juniper MX here when the TPID is 0x8100 please let us know.