Cisco Systems 15327 E-Series Priority Queuing (IEEE 802.1Q)

23-20
Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 23 E-Series and G-Series Ethernet Operation
E-Series Priority Queuing (IEEE 802.1Q)
E-Series Priority Queuing (IEEE 802.1Q)
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 E thernet traffic to different priority
levels. The E-Series card supports priority queuing. The E-Series card maps the eight priorities specified
in IEEE 802.1Q to two queues, low priority and high priority (Table 23-3).
Q-tags carry priority queuing information through the network (Figure 23-16).
Figure 23-16 Priority Queuing Process
The ONS node uses a “leaky bucket” algorithm to establish a weighted priority. A weighted priority, as
opposed to a strict priority, gives high-priority packets greater access to bandwidth, but does not totally
preempt low-priority packets. During periods of network congestion, abou t 70 percent of bandwidth
goes to the high-priority queue and the remaining 30 percent goes to the low-priority queue. A network
that is too congested will drop packets.
Table 23-3 Priority Queuing
User Priority Queue Allocated Bandwidth
0,1,2,3 Low 30%
4,5,6,7 High 70%
Data Flow
No priority
ONS node maps a frame
with port-based priority using
a Q-tag.
The receiving ONS node
removes the Q-tag and
forwards the frame.
ONS node uses a Q-tag to
map a frame with priority and
forwards it on.
The receiving ONS node
receives the frame with a
Q-tag and forwards it.
Priority tag
removed
Priority Priority
Priority
Same
priority
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