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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?

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What is the functional difference between these two TPID values?

0x8100

This TPID is used in 802.1Q - its your average, everyday VLAN tagging. A common use for VLANs is for enterprises to organize their network into functional groups (like research, finance, or management).

0x88a8

This TPID is used in 802.1ad - its used for provider bridging (also known as QinQ, stacked VLANs, or double tagging). QinQ allows multiple VLAN tags in an Ethernet frame.

QinQ is used when a customer has to transport VLAN tagged traffic across a service provider network. The service provider will have its own set of VLAN tags, perhaps a tag per customer. So we have customer VLAN tags, and service provider VLAN tags, appropriately called C-TAGs and S-TAGs.

S-TAGs are correlated with the 0x88a8 TPID to signify the existence of the inner C-TAG which uses TPID 0x8100 (S-TAGs are inserted before C-TAGs).

Why would Juniper use a different default TPID than the IEEE reserved TPID for the S-TAG?

They're not, here is a list of the common IEEE TPIDs.

To be more specific, the default is 0x8100 because 802.1Q is used WAY more commonly than 802.1ad/Provider Bridging.

To be even more specific to what I assume is a quote from the JNCIS documentation, is just really poor wording. I believe it's saying that the default TPID is 0x8100 (to imply that 802.1q is default).

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    Yes, in short 0x8100 is understood by almost everything in the universe, which makes it a safe default. If your hardware supports 0x88a8, then you can set it to be used. Technically, the dtag tpid can be set to anything -- at least with broadcom tech. I've set it to 0x9100 intentionally to stop 802.1q aware hardware from screwing with the traffic. (under lab conditions, of course) – Ricky Jun 17 '15 at 19:33
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    Also to note, you can use 0x8100 for multiple tags. You don't have to use 0x88a8 for multiple tagging. The provider that I work for has layer 1/2 providers that connect to us that double/triple tag using both methods. I honestly don't see a benefit of one of the other. – Firebirdnz Jun 18 '15 at 2:42
  • @Firebirdnz the benefit of using 88a8 is in the case where some traffic might be S-tagged and other traffic might not be S-tagged but might be C-tagged. With clever tagging it's possible to dig yourself into a nice deep hole. – Jeff Learman Nov 3 '20 at 16:23

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