Another type of overlay networking is QinQ tunnelling.
QinQ involves use multiple VLAN tags in an Ethernet header so that one VLAN ID can carry another 4096 VLAN IDs in a second tag. This makes a simple and useful tunnelling strategy.
The first/inner tag is the one set by the customer, and the second/outer tag would be set by the network. It’s common in the Service Provider industry 1 to refer to these as Customer VLAN and Service VLAN
| Tag Location | Tag Name | VLAN Name | VLAN Abbreviation / Acronym |
|---|---|---|---|
| Inner Tag | Customer Tag | Customer VLAN | cVLAN |
| Outer Tag | Service Tag | Service VLAN | sVLAN |
- who are the only people who can deploy QinQ reliably since they don’t have to handle the loops – that’s still a customer problem. ↩
This whole c-tag/s-tag doohickie seems to cause serious confusion due to the nomenclature and the tags positioning in a frame relative to each other.
The only difference I know between the two tags is the CFI bit in one is the DEI bit in the other. Vendors seem to use the same Ethertype so there isn’t a huge amount of distinction between the two.
From a Cisco 7600:
7609#sho ethernet ser ins de | i second-d
Encapsulation: dot1q 224 vlan protocol type 0x8100 second-dot1q 800 vlan protocol type 0x8100
And an ALU 7750:
A:7750>config>port# show port 6/2/10 | match Ethert
Dot1Q Ethertype : 0x8100 QinQ Ethertype : 0x8100
Both the above are default configurations. 802.1ad points to using an ethertype of 0x88a8 irc so vendors dont seem to default to the standard.
Whats my point here? If an untagged frame has an outer tag pushed by the provider, which is configured as an s-tag on the NTE/edge switch, does the s-tag become the c-tag as it is the only tag in the frame? And given the only difference is the drop eligibility indicator how are you supposed to distinguish the two if both/neither the CFI or DEI bits are set, say in a sniffer. Wireshark does identify double tagged as dot1q and q-in-q in the header, but I dont think it says a frame is an s-tag/q-in-q if its the only tag within the header.
Sorry for the rambling, the earth has little time left to live..