Cisco 15327 User Manual download pdf (Page 400)

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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)

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 (Tabl e 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, 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

Same

priority

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