Design Reference

Network congestion and QoS design
When you provide QoS in a network, one of the major elements you must consider is congestion,
and the traffic management behavior during congestion. Congestion in a network is caused by many
different conditions and events, including node failures, link outages, broadcast storms, and user
traffic bursts.
At a high level, three main types or stages of congestion exist:
1. No congestion
2. Bursty congestion
3. Severe congestion
In a noncongested network, QoS actions ensure that delay-sensitive applications, such as real-time
voice and video traffic, are sent before lower-priority traffic. The prioritization of delay-sensitive traffic
is essential to minimize delay and reduce or eliminate jitter, which has a detrimental impact on these
applications.
A network can experience momentary bursts of congestion for various reasons, such as network
failures, rerouting, and broadcast storms. Avaya Virtual Services Platform 4000 Series has sufficient
capacity to handle bursts of congestion in a seamless and transparent manner. If the burst is not
sustained, the traffic management and buffering process on the switch allows all the traffic to pass
without loss.
Severe congestion is defined as a condition where the network or certain elements of the network
experience a prolonged period of sustained congestion. Under such congestion conditions,
congestion thresholds are reached, buffers overflow, and a substantial amount of traffic is lost.
After the switch detects severe congestion, Avaya Virtual Services Platform 4000 Series discards
traffic based on drop precedence values. This mode of operation ensures that high-priority traffic is
not discarded before lower-priority traffic.
When you perform traffic engineering and link capacity analysis for a network, the standard design
rule is to design the network links and trunks for a maximum average-peak utilization of no more
than 80%. This value means that the network peaks to up to 100% capacity, but the average-peak
utilization does not exceed 80%. The network is expected to handle momentary peaks above 100%
capacity.
QoS examples and recommendations
The sections that follow present QoS network scenarios for bridged and routed traffic over the core
network.
Bridged traffic
If you bridge traffic over the core network, you keep customer VLANs separate (similar to a Virtual
Private Network). Normally, a service provider implements VLAN bridging (Layer 2) and no routing.
In this case, the 802.1p-bit marking determines the QoS level assigned to each packet. If DiffServ is
QoS design guidelines
166 Network Design Reference for Avaya VSP 4000 Series June 2015
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