Cisco Systems 15454M6DC 9.4.2 Priority Queuing (IEEE 802.1Q)

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Cisco ONS 15454 Installation and Operations Guide
November 2001
Chapter9 Ethernet Operation
VLAN Support
Figure9-20 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 Eth er net tra ffic to differe nt p rior ity le ve ls. T he
ONS 15454 supports priority queuing. The ONS 15454 takes the eight priorities specified in IEEE
802.1Q and maps them to two queues (Tab le 9-5). Q-tags carry prio rity 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 pre empt
low-priority packets. During periods of network congestion, ro ugh ly 70 % o f bandw idt h goe s to t he
high-priority queue and the remaining 30% goes to the low-priori ty queue . A ne tw ork t hat 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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