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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
Distributed-Platform High-Speed Frame Relay Links
Most policies, when ported to distributed platforms (such as the Cisco 7500 VIP), require little more than
ensuring that CEF is running in distributed mode. However, because FRTS is not supported in a
distributed environment, another shaping tool is required. Distributed Traffic Shaping (dTS) can be used
in conjunction with hierarchical MQC policies to achieve a similar effect on traffic flows over
distributed Frame Relay WAN links.
However, the Cisco 7500 VIP requires the interval (Tc) to be defined in an increment of 4 ms. Because
the target interval for all platforms is 10 ms, which is not evenly divisible by 4 ms, the recommendation
for Cisco 7500 VIP is to use an interval of 8 ms. The interval can be set to 8 ms by defining the burst
using the following formula:
Bc = CIR/125
For example, for a T1 (1.536 Mbps), the Bc for an 8 ms interval is 12288 bits.
As before, the Be is set to 0. Additionally, it is recommended that you increase the queue-limit from the
default of 126 packets to reduce the potential for tail-drops from the shaping queue. To provision the
shaping queue to allow for shaping for up to a 10 second period, use the following formula:
Queue-limit = (CIR * 10) / (MTU * 8)
For example, the queue-limit for a T1 circuit (with an MTU of 1500) would be calculated as:
Queue-limit = (1536000*10)/(1500*8) or 1280 packets
Tip
For more information, see Configuring Distributed Traffic Shaping.
illustrates using dTS and hierarchical MQC policies for a distributed-platform Frame Relay
WAN aggregator.
Example 4-8
Distributed-Platform High-Speed Frame Relay Link
ip cef distributed
!
...
!
policy-map dTS-1536kbps
class class-default
shape average 1536000 12288 0
This command is the dTS shaper (CIR at T1 rate, Bc=CIR/125).
queue-limit 1280
This command increases the queue limit.
service-policy WAN-EDGE
This command applies the LLQ/CBWFQ policies within the dTS
!
policy.
interface Serial1/0/1
bandwidth 1536
no ip address
encapsulation frame-relay
no fair-queue
!
interface Serial1/0/1.50 point-to-point
description FR Link to BRANCH#50
bandwidth 1536
ip address 10.200.50.1 255.255.255.252
frame-relay interface-dlci 150
class dTS-TO-FRTS-1536kbps
This command binds the Frame Relay map-class to the DLCI.
!
...