Enterprise QoS Solution Reference Network Design Guide

Chapter 3 WAN Aggregator QoS Design
Version 3.3
WAN Edge Classification and Provisioning Models
Voice is identified by DSCP EF, which is set by default on Cisco IP phones. When identified, VoIP is
admitted into the LLQ, which, in this example, is set to the maximum recommended value of 33 percent
of the link. Call admission control (CAC) correspondingly should be assigned to this link by dividing
the allocated bandwidth by the voice codec (including Layer 2 overhead) to determine how many calls
can be permitted simultaneously over this link. Because class-based cRTP is used in this example to
compress voice traffic, it also should be factored into the CAC calculation.
Call-Signaling traffic also is marked on the IP phones (to AF31 currently, but it will be migrated to CS3,
per the QoS Baseline) and requires a relatively small but dedicated bandwidth guarantee. All other data
is fair-queued within class-default. This Three-class WAN Edge Model is illustrated in Figure 3-2 and
detailed in Example 3-4.
Figure 3-2 Three-Class WAN Edge Model Migration Strategy Example
Best-Effort
(62%)
Example 3-4 Three-Class WAN Edge Model
!
class-map match-all Voice
match ip dscp ef ! IP Phones mark Voice to EF
class-map match-any Call Signaling
match ip dscp cs3 ! Future Call-Signaling marking
match ip dscp af31 ! IP Phones mark Call-Signaling to AF31
!
policy-map WAN-EDGE
class Voice
priority percent 33 ! Maximum recommended LLQ value
compress header ip rtp ! Optional: Enables Class-Based cRTP
class Call Signaling
bandwidth percent 5 ! BW guarantee for Call-Signaling
class class-default
fair-queue ! All other data gets fair-queuing
!
Sometimes administrators explicitly create a class map that functions as the MQC class-default. For
instance, an administrator might create a class along the lines of that shown in the following code:
class-map match-all BEST-EFFORT
match any
or even:
class-map match-all BEST-EFFORT
match access-group 101
...
access-list 101 permit ip any any
Voice
33%
Call-Signaling
5%
Enterprise QoS Solution Reference Network Design Guide
3-7