E-series priority queuing (ieee 802.1q), E-series priority queuing, Figure 23-16 – Cisco 15327 User Manual
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23-20
Ethernet Card Software Feature and Configuration Guide, R7.2
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 Ethernet 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 (
Q-tags carry priority queuing information through the network (
).
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, about 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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