The provider edge bridge is split into two kinds of components,
the S-components and the C-components. Each is, in theory, simply an 802.1Q
VLAN bridge. The C-component is exactly the same as the customer??™s 802.1Q VLAN
bridge. The S-component is identical, except that it uses the S-tag instead of the C-tag.
As far as the S-component is concerned, C-tags do not exist; should it receive one, it
considers it simply an untagged frame with an unknown Ethertype.
The C-components that link to the S-component certainly can be physical, but they
are usually virtual. In the example shown in Figure 13.7, the upper C-component is
configured to pass C-VIDs 1, 2, and 40 over virtual link 1, and C-VID 18 only on virtual
link 2. Also, this C-component is configured so its port on virtual link 2 emits its frames
untagged, and when untagged frames are received, they are assigned to C-VLAN 18.
Figure 13.7 Provider bridge C-tagged interface
Customer 1
Customer 2
C-components S-component S-component
Provider edge bridge Provider bridge
Virtual
links
C-tagged
frames
S-10
S-11
S-20
S-21
C-1,2,40
C-18
(untagged)
odd C??™s
even C??™s
S-tagged
frames S-tagged
frames
Customer equipment
1
2
3
4
Ethernet Bridging 391
The lower C-component delivers all of the odd-numbered C-VIDs to virtual link 3 and
the even-numbered C-VIDs to virtual link 4.
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