The S-tag is shown in Figure 13.4d. It is identical to the C-tag except for the EtherType
(0 ?— 88A8 instead of 0 ?— 8100) and the Drop Eligible Indicator (DEI) bit in place of the
CFI bit. The DEI bit and its associated implementation inside the bridge provide two
levels of delivery priority on top of the eight priority levels provided by the C-tag. The
DEI is explained in ???Rate Policing,??? later in this chapter.
On the typical provider bridge port, the EtherType used to recognize a VLAN tag is
the S-tag value 0 ?— 88A8. A frame tagged (by the customers??™ equipment) with a C-tag
(EtherType 0 ?— 8100) is, therefore, as far as the provider bridge??™s receiving port is concerned,
untagged. (The exception to this rule is the C-tagged interface, described next.)
The received frame is assigned the PVID for that port. You may assume, for the moment,
Figure 13.4 S-tag frame format for 802.1ad provider bridges
destination_address source_address Type/
length
data FRC
destination_address source_address
0x88A8 (was 0x8100 in C-tag) Priority
D
E
I
S-VLAN ID (S-VID)
6 6
6
2
2 4
46-1498 4
Type/
length
data FRC
46-1498 4 6
16 3 1 12
C-VLAN tag
(optional)
(a) Untagged
frame
(b) VLAN-
tagged frame
(d) Format of
S-VLAN tag
bits
bytes
bytes
destination_address source_address Type/
length
data FRC
2 46-1498 4 6 6 4
C-VLAN tag
(optional)
(c) S-tagged
frame
bytes
S-VLAN tag
(was CFI in C-tag)
Ethernet Bridging 385
that each customer is assigned to a different VLAN and that VLAN??™s ID is the PVID for
each port connected to that customer.
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