Explanation/Reference:
+ classification and marking. ACLs
+ congestion avoidance. WRED
+ traffic conditioners: CAR
+ congestion management: LLQ
+ link efficiency: LFI
Classication is the process of partitioning trafc into multiple priority levels or classes of service. Information
in the frame or packet header is inspected, and the frame's priority is determined.Marking is the process of
changing the priority or class of service (CoS) setting within a frame or packet to indicate its classication.
Classication is usually performed with access control lists (ACL), QoS class maps, or route maps, using
various match criteria.
Congestion-avoidance techniques monitor network trafc loads so that congestion can be anticipated and
avoided before it becomes problematic. Congestion-avoidance techniques allow
packets from streams identied as being eligible for early discard (those with lower priority) to be dropped
when the queue is getting full. Congestion avoidance techniques provide preferential treatment for high
priority trafc under congestion situations while maximizing network throughput and capacity utilization and
minimizing packet loss and delay.
Weighted random early detection (WRED) is the Cisco implementation of the random early detection
(RED) mechanism. WRED extends RED by using the IP Precedence bits in the IP
packet header to determine which trafc should be dropped; the drop-selection process is weighted by the
IP precedence.
Traffic conditioner consists of policing and shaping. Policing either discards the packet or modies some
aspect of it, such as its IP Precedence or CoS bits, when the policing agent determines that the packet
meets a given criterion. In comparison, trafc shaping attempts to adjust the transmission rate of packets
that match a certain criterion. Shaper typically delays excess trafc by using a buffer or queuing mechanism
to hold packets and shape the ow when the source's data rate is higher than expected. For example,
generic trafc shaping uses a weighted fair queue to delay packets to shape the bw. Traffic conditioner is
also referred to as Committed Access Rate (CAR).
Congestion management includes two separate processes: queuing, which separates trafc into various
queues or buffers, and scheduling, which decides from which queue trafc is to be sent next. There are two
types of queues: the hardware queue (also called the transmit queue or TxQ) and software queues.
Software queues schedule packets into the hardware queue based on the QoS requirements and include
the following types: weighted fair queuing (WFQ), priority queuing (PQ), custom queuing (CQ), class-
based WFQ (CBWFQ), and low latency queuing (LLQ).
LLQ is also known as Priority Queuing-Class-Based Weighted Fair Queuing (PQ-CBWFQ). LLQ provides
a single priority but it's preferred for VoIP networks because it can also congure
guaranteed bandwidth for different classes of trafc queue. For example, all voice call trafc would be
assigned to the priority queue, VoIP signaling and video would be assigned to a trafc class, FTP trafc
would be assigned to a low-priority trafc class, and all other trafc would be assigned to a regular class.
Link efciency techniques, including link fragmentation and interleaving (LFI) and compression. LFI
prevents small voice packets from being queued behind large data packets, which could lead to
unacceptable delays on low-speed links. With LFI, the voice gateway fragments large packets into smaller
equal-sized frames and interleaves them with small voice packets so that a voice packet does not have to
wait until the entire large data packet is sent. LFI reduces and ensures a more predictable voice delay.
(Reference. Cisco Press Designing for Cisco Internetwork Solutions)