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B-8
Cisco 860 Series, Cisco 880 Series, and Cisco 890 Series Integrated Services Routers Software Configuration Guide
OL-18906-02
Appendix B Concepts
QoS
IP Precedence
You can partition traffic in up to six classes of service using IP Precedence (two others classes are
reserved for internal network use). The queuing technologies throughout the network can then use this
signal to expedite handling.
Features such as policy-based routing and committed access rate (CAR) can be used to set precedence
based on extended access-list classification. This allows considerable flexibility for precedence
assignment, including assignment by application or user, by destination and source subnet, and so on.
Typically this functionality is deployed as close to the edge of the network (or administrative domain)
as possible, so that each subsequent network element can provide service based on the determined policy.
IP Precedence can also be set in the host or network client with the signaling used optionally. IP
Precedence enables service classes to be established using existing network queuing mechanisms (such
as class-based weighted fair queueing [CBWFQ]) with no changes to existing applications or
complicated network requirements.
PPP Fragmentation and Interleaving
With multiclass multilink PPP interleaving, large packets can be multilink-encapsulated and fragmented
into smaller packets to satisfy the delay requirements of real-time voice traffic; small real-time packets,
which are not multilink encapsulated, are transmitted between fragments of the large packets. The
interleaving feature also provides a special transmit queue for the smaller, delay-sensitive packets,
enabling them to be transmitted earlier than other flows. Interleaving provides the delay bounds for
delay-sensitive voice packets on a slow link that is used for other best-effort traffic.
In general, multilink PPP with interleaving is used in conjunction with CBWFQ and RSVP or IP
Precedence to ensure voice packet delivery. Use multilink PPP with interleaving and CBWFQ to define
how data is managed; use Resource Reservation Protocol (RSVP) or IP Precedence to give priority to
voice packets.
CBWFQ
In general, class-based weighted fair queuing (CBWFQ) is used in conjunction with multilink PPP and
interleaving and RSVP or IP Precedence to ensure voice packet delivery. CBWFQ is used with multilink
PPP to define how data is managed; RSVP or IP Precedence is used to give priority to voice packets.
There are two levels of queuing; ATM queues and Cisco IOS queues. CBWFQ is applied to Cisco IOS
queues. A first-in-first-out (FIFO) Cisco IOS queue is automatically created when a PVC is created. If
you use CBWFQ to create classes and attach them to a PVC, a queue is created for each class.
CBWFQ ensures that queues have sufficient bandwidth and that traffic gets predictable service.
Low-volume traffic streams are preferred; high-volume traffic streams share the remaining capacity,
obtaining equal or proportional bandwidth.
RSVP
RSVP enables routers to reserve enough bandwidth on an interface to ensure reliability and quality
performance. RSVP allows end systems to request a particular QoS from the network. Real-time voice
traffic requires network consistency. Without consistent QoS, real-time traffic can experience jitter,