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4-25
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
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Chapter 4 QoS in an AVVID-Enabled Wide-Area Network
QoS Recommendations for WAN Aggregation Routers
branch goes down and ISDN backup engages, the Call Manager remains ignorant of the occurrence. For
this reason, it is critical that the ISDN backup link can handle the same number of VoIP calls as the main
link. Otherwise, CAC could ultimately oversubscribe the backup link.
The actual bandwidth of the primary link and the backup link do not need to be identical. They just need
to be able to carry the same number of VoIP calls. For example, the backup link may use cRTP while
the primary link does not, in which case less bandwidth is required on the backup link to carry the same
number of calls as the primary link.
Because of these limitations, it is recommended that the 33% LLQ ceiling be relaxed in this dial-backup
scenario. The LLQ could be provisioned as high as 73% (leaving 2% for Voice Control traffic over the
ISDN link).
Voice and Data on Multiple ISDN B Channels
This design takes advantage of the fact that LLQ bandwidth can be expressed as a percentage instead of
an absolute number. This allows a service policy to be applied to a bundle with multiple B channels. As
a result, RTP packets can be carried across multiple B channels. The drawback is that cRTP will not
work properly in such a configuration, due to the potential reordering of RTP packets.
IOS provides two mechanisms for controlling how channels are added in response to demand.
·
The first mechanism is commonly referred to as Dial on Demand Routing (DDR). With DDR, a
load-threshold must be specified (as fraction of available bandwidth). When traffic load exceeds this
number, an additional channel is added to the bundle. The threshold is calculated as a running
average. Therefore, there is certain delay in bringing up additional B channels when the load
increases. This delay does not present a problem with data, but with voice it is unacceptable. This
delay can be reduced to around 30 seconds by adding the load-interval command to the physical
ISDN interface, but even 30 seconds is too long.
·
The second is a more robust solution, which is to simply bring all B channels up immediately and
keep them up as long as the ISDN service is required. This is achieved by using the ppp multilink
links minimum
command.
With two B channels available, the service policy can now reserve 93 kbps (73% of 128 kbps) for
voice and voice control traffic. The total number of calls that can be transmitted will depend on the
CODEC and sampling rates used.
Example 4-16
illustrates the configuration for enabling voice and data over multiple ISDN B channels.
Example 4-16 Voice and Data over Multiple B Channels
class-map match-all VOICE
match ip dscp ef
!
class-map match-all VOICE-CONTROL
match ip dscp af31
!
policy-map WAN-EDGE-ISDN
class VOICE
priority percent 73
This command relaxes the LLQ ceiling for this DDR scenario. (73% of 128
class VOICE-CONTROL
kbps is 93 kbps. That leaves 2%, which is 2.5 kbps; enough for 15 phones.)
bandwidth percent 2
class class-default
fair-queue
!
interface BRI0/0
encapsulation ppp
dialer pool-member 1
!