A provider bridge, however, is different. It uses 01-80-C2-
00-00-08 for its BPDUs and forwards frames with destination MAC addresses ??¦-00,
??¦-0A, ??¦-0B, ??¦-0C, and ??¦-0F through the provider bridge just like any other ordinary
multicast frame. The customers??™ BPDUs are thus just like any other multicast data to
the provider bridge.
Rate Policing In the carrier environment, it may be convenient for the provider to supply
an interface that works at a higher physical rate than the bit rate offered the customer,
especially since standard IEEE 802.3 Ethernet link speeds are whole powers of
388 Chapter 13
10??“10 Mbps, 100 Mbps, 1 Gbps, and so on. The simple rate policing offered by a provider
bridge simply discards all frames in excess of the customer??™s guarantee. However, more sophisticated
control is made possible by the Drop Eligible Indicator (DEI) bit in the S-tag.
The need for the DEI bit can be illustrated by a provider/customer service-level agreement
(SLA) that specifies that the provider guarantees the customer can transfer 15 Mbps
and can transfer up to 50 Mbps on a best-effort basis as the provider??™s network bandwidth
allows.
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