Cisco ONS 15327, User Documentation

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Cisco ONS 15327 User Documentation, R3.3
June 2002
Chapter 9 Ethernet Operation
VLAN Support
Networks without priority queuing handle all packets on a first-in, first-out (FIFO) basis. Priority
queuing reduces the impact of network congestion by mapping Ethernet traffic to different priority
levels. The ONS 15327 supports priority queuing. The ONS 15327 takes the eight priorities specified in
IEEE 802.1Q and maps them to two queues (
Table 9-3 ). Q-tags carry priority queuing information
through the network.
The ONS 15327 uses a “leaky bucket” algorithm to establish a weighted priority (not a strict priority).
A weighted priority gives high-priority packets greater access to bandwidth, but does not totally preempt
low-priority packets. During periods of network congestion, roughly 70% of bandwidth goes to the
high-priority queue and the remaining 30% goes to the low-priority queue. A network that is too
congested drops packets.
Figure 9-21 Priority queuing process
Table 9-3 Priority Queuing
User Priority Queue Allocated Bandwidth
0,1,2,3 Low 30%
4,5,6,7 High 70%
Data Flow
No priority
ONS 15327
ONS 15327
ONS 15454 maps a frame
with port-based priority using
a Q-tag. The receiving ONS 15327
removes the Q-tag and
forwards the frame.
ONS 15327 uses a Q-tag to
map a frame with priority and
forwards it on. The receiving ONS 15327
receives the frame with a
Q-tag and forwards it.
Priority tag
removed
Priority
Priority
Priority
Same
priority
ONS 15327
ONS 15327
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