1Q-2005; however, these assignments can be overridden
by management.
A bridge may provide other queue-draining algorithms as well. Practically every
vendor that supplies bridges with multiple queues offers the capability of limiting the
share of bandwidth available to each priority level, so that a long burst of high-priority
traffic does not lock out all lower priority traffic. Although the field is called priority
field, it is really a class-of-service indicator.
Provider Bridges??”Q-in-Q
As soon as providers started using VLANs to carry different customers??™ data, the question
arose, ???What if customers are already using VLANs???? Several ad hoc solutions
were tried by various vendors, and these led eventually to IEEE Std 802.1ad-2005 for
provider bridges.
An 802.1ad provider bridge is simply an 802.1Q VLAN bridge with five modifications:
?– A provider bridge uses a slightly different format for its VLAN tag (see Figure 13.4.)
?– A provider bridge blocks a reduced set of destination multicast MAC addresses. Of
the 16 addresses never forwarded by a non-VLAN bridge, a provider bridge still
blocks 11 of them, but forwards 5 of them transparently.
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