High-Priority Classes; Assured Classes - Alcatel-Lucent 7950 Quality Of Service Manual

The forwarding classes can be classified into three class types:
•
•
•

High-Priority Classes

The high-priority forwarding classes are Network Control (nc), Expedited (ef), High 1 (h1), and
High 2 (h2). High-priority forwarding classes are always serviced at congestion points over other
forwarding classes; this behavior is determined by the router queue scheduling algorithm
Hierarchical Scheduling on page
With a strict PHB at each network hop, service latency is mainly affected by the amount of high-
priority traffic at each hop. These classes are intended to be used for network control traffic or for
delay or jitter-sensitive services.
If the service core network is over-subscribed, a mechanism to traffic engineer a path through the
core network and reserve bandwidth must be used to apply strict control over the delay and
bandwidth requirements of high-priority traffic. In the router, RSVP-TE can be used to create a
path defined by an MPLS LSP through the core. Premium services are then mapped to the LSP
with care exercised to not oversubscribe the reserved bandwidth.
If the core network has sufficient bandwidth, it is possible to effectively support the delay and
jitter characteristics of high-priority traffic without utilizing traffic engineered paths, as long as the
core treats high-priority traffic with the proper PHB.

Assured Classes

The assured forwarding classes are Assured (af) and Low 1 (l1). Assured forwarding classes
provide services with a committed rate and a peak rate much like Frame Relay. Packets transmitted
through the queue at or below the committed transmission rate are marked in-profile. If the core
service network has sufficient bandwidth along the path for the assured traffic, all aggregate in-
profile service packets will reach the service destination. Packets transmitted out the service queue
that are above the committed rate will be marked out-of-profile. When an assured out-of-profile
service packet is received at a congestion point in the network, it will be discarded before in-
profile assured service packets.
Multiple assured classes are supported with relative weighting between them. In DiffServ, the code
points for the various Assured classes are AF4, AF3, AF2 and AF1. Typically, AF4 has the highest
weight of the four and AF1 the lowest. The Assured and Low 1 classes are differentiated based on
the default DSCP mappings. Note that all DSCP and EXP mappings can be modified by the user.
7950 XRS Quality of Service Guide
High-priority/Premium
Assured
Best effort
56).
QoS Policies
(Virtual
Page 65