1731)???
in every CFM/OAM message to differentiate the levels. Thus, in Figure 13.15, the endto-
end provider cannot see either operators??™ network except at the four points where the
end-to-end EVC enters and leaves the operators??™ networks. Similarly, the customer can
see only the entry and exit points of the end-to-end service.
At each level, the visible intermediate points are the endpoints of the lower level. In
addition to essential information hiding, this model provides an excellent means for
reporting and diagnosing faults. The endpoints of each level can report failures to the
enclosing level. If a failure is detected at any given level, it can be isolated to that part
of the network between intermediate points, and the appropriate subnetwork can then
be inspected to isolate the difficulty further.
Figure 13.15 Connectivity fault management
Customer
equipment
Operator 1
provider bridges
Operator 2
provider bridges
Customer
equipment
Only four points visible to customer
Four others visible to provider
Each operator sees only his own points
Physical links controlled via CFM/OAM, hardware, MPLS, or SONET
Ethernet Bridging 411
As a final note, you may observe that the only thing guaranteed to be transported
through a Carrier Ethernet network end to end, no matter what technology (SONET,
MPLS, bridges, ATM, or anything else) is used in the network, is an Ethernet frame.
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