The
principles of bridge gateways are firmly established and have been implemented by some
vendors, though their standardization is still in progress (as part of IEEE P802.1ah).
The bridge gateway is based on the following principles:
?– There are two separate spanning tree domains. There may be any number of pointto-
point links between the domains. (Shared media connections can be handled,
but require some additional effort.)
?– One bridge, or a cloud of bridges, makes all of the decisions as to which of the links
are used for which VLANs. This is the ???client cloud.???
Figure 13.13 Bridge gateways
Customer network 1 Provider network Customer network 2
Client cloud Server cloud Client cloud
A
B
C
D
E
F
G
H
odd odd
even even
Ethernet Bridging 405
?– The other bridge, or a cloud of bridges, is needed to deliver all of the VLANs on
every port connected to the client cloud. This is the ???server cloud.???
?– The client cloud may inform the server cloud about which VLANs are passed on
each link in order to minimize wasted bandwidth.
On the server cloud side, the bridge ports connected to a client cloud are configured to
not transmit BPDUs and to discard them on receipt.
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