Cisco Systems 15454 SDH 14.3.5 E-Series Priority Queuing (IEEE 802.1Q)

REVIEW DRAFT—CISCO CONFIDENTIAL
14-15
Cisco ONS 15454 SDH Reference Manual, R5.0
April 2008
Chapter14 Ethernet Operation
14.3.5 E-Series Priority Queuing (IEEE 802.1Q)
14.3.5 E-Series Priority Queuing (IEEE 802.1Q)
Networks without priority queuing handle all packets on a FIFO basis. Priority queuing reduces the
impact of network congestion by mapping Ethernet traffic to different priority levels. The ONS 15454
SDH supports priority queuing. The ONS 15454 SDH maps the eight priorities specified in IEEE 802.1Q
to two queues, low priority and high priority number (Table 14 -1).
Q-tags carry priority queuing information through the ne twork (Figure 14-13).
Figure14-13 Priority Queuing Process
The ONS 15454 SDH 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 percent of
bandwidth goes to the high-priority queue and the remaining 30 percent goes to the low-priority que ue.
A network that is too congested will drop packets.
Table14-1 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
61076