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Cisco ONS 15454 Installation and Operations Guide
November 2001
Chapter 9 Ethernet Operation
VLAN Support
Figure 9-20 A Q-tag moving through a VLAN
9.4.2 Priority Queuing (IEEE 802.1Q)
Note IEEE 802.1Q was formerly IEEE 802.1P.
Networks without priority queuing handle all packets on a first-in-first-out basis. Priority queuing
reduces the impact of network congestion by mapping Ethernet traffic to different priority levels. The
ONS 15454 supports priority queuing. The ONS 15454 takes the eight priorities specified in IEEE
802.1Q and maps them to two queues (Table 9-5). Q-tags carry priority queuing information through the
network.
The ONS 15454 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 will drop packets.
Data Flow
No tag
ONS 15454
ONS 15454
ONS 15454
ONS 15454
1. The
ONS 15454
uses a Q-tag internally
to deliver the frame to a
specific VLAN.
The receiving
ONS 15454
removes the Q-tag
and forwards the frame
to the specific VLAN.
2. The
ONS 15454
receives a frame with a
Q-tag and passes it on.
The receiving
ONS 15454
receives a frame with a
Q-tag and passes it on.
No tag
Q-tag Q-tag
Q-tag
Q-tag
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