Understanding the network.pdf - Back to Home

The third reason is perhaps an extension of the first and second. It has
to do with the poor initial performance of most of the proprietary
VLAN implementations. When the first store-and-forward switches
with VLAN capability came on the market, many of them had the
software functionality, but did not have switching engines powerful
enough to process the VLAN packet flows efficiently. This lackluster
performance early in the development of VLANs burned many of the
initial buyers of the technology. The real irony today is watching the
same issues come up again with the development of Gigabit Ethernet.
The idea is solid, but the implementation falls short.
802.1p Layer 2 Traffic Prioritization
802.1p defines a standard for implementing Layer 2 CoS traffic
prioritization. It provides the capability for end-stations (equipped
with 802.1p-compatible NICs) and servers to set traffic prioritization
based on traffic type, port, or VLAN. 802.1p provides eight different
traffic classes (see Table 6.1) to specify different prioritization levels:
0 through 7, with 7 being the highest.
Table 6.1. The IEEE 802.1p Traffic Class Recommendations
User Priority Bits Traffic Priority Class
111 Network Critical (7)
110 Interactive Voice (6)
101 Interactive Multimedia (5)
100 Streaming Multimedia (4)
011 Business Critical (3)
010 Standard (2)
001 Background (1)
000 Best Effort (0)
The 802.1p standard specifies a 3-bit header to indicate the packet's
priority class. This header is part of the 802.1q 32-bit VLAN tag in the
form of the user priority field. So, although 802.1p and 802.1q are
defined as separate standards, this tandem relationship effectively
makes 802.1p and 802.1q almost mutually exclusive to one another
in terms of implementation. In order for the incoming packets to be
placed in the appropriate delivery queue, both the end-station and the
switch need to be able to discern different types of traffic when