Thus, only one bridge in the
provider??™s network will be learning that customer??™s MAC addresses. Proper alignment
of multiple spanning trees can ensure that the learning load is spread properly around
the network to reduce the need for MAC address learning.
See ???Backbone Bridges??”Mac-in-Mac,??? for a more complete solution to the MAC address
problem.
Number of VLANs The limit of 4094 VLANs is set by the size of the VLAN ID field in
the Q-tag. It has 12 bits, allowing 4096 combinations; 0 means ???I don??™t know??? and 4095
is illegal, leaving 4094 useful VLANs. This may not be enough.
One big reason for the existence of Carrier Ethernet networks is the connection of
Internet service providers (ISPs) to their subscribers. The most obvious way to connect
Ethernet Bridging 393
is to dedicate a VLAN for each subscriber and bring the VLANs to the ISP??™s router. This
solution is inadequate for more than 4094 subscribers.
Many ISPs have, for years, used a solution often called private VLANs for this problem.
Instead of using one VLAN per subscriber, a Carrier Ethernet network can use two
VLANs for any number of subscribers, say 500.
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