Enterprise QoS Solution Reference Network Design Guide
Enterprise QoS Solution Reference Network Design Guide
Enterprise QoS Solution Reference Network Design Guide
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Chapter 2 Campus <strong>QoS</strong> <strong>Design</strong><br />
Version 3.3<br />
WAN Aggregator/Branch Router Handoff Considerations<br />
For example, a WAN Aggregation router may support two DS3 WAN connections (totaling 90 Mbps of<br />
WAN circuit-capacity). In this case, the Distribution Layer switch port connecting to the WAG should<br />
be FastEthernet. Then, if more than 100 Mbps of traffic attempts to traverse the WAN, the Catalyst<br />
switch engages queuing on the switch port and aggressively drops flows according to the defined<br />
application hierarchies. Only 100 Mbps of correctly-queued traffic is ever handed off to the WAG.<br />
In the case of a WAG supporting over 100 Mbps of WAN circuits, like the case of a WAG running one<br />
or more OC-3 ports (at 155 Mbps each), then multiple FastEthernet connections can be used to connect<br />
to the WAG from the Distribution Layer switch to achieve the same net effect.<br />
The point is to bring back, as much as possible, the choke point into Catalyst hardware and engage<br />
hardware queuing there, rather than overwhelming the software-based policing and/or queueing policies<br />
within the WAG.<br />
Second, if the combined WAN circuit-rate is significantly below 100 Mbps, enable egress shaping<br />
on the Catalyst switches (when supported).<br />
If there is no hope of engaging queuing on the Catalyst switch because the combined WAN circuit-rates<br />
are far below FastEthernet (the minimum port speed of Catalyst switches), then enable shaping on<br />
platforms that support this feature. Such platforms include the Catalyst 2970, 3560, 3750, and 4500.<br />
In this manner, the Catalyst switch can hold back traffic and selectively drop (according to defined<br />
policies) from flows that would otherwise flood the WAN/Branch router.<br />
For example, if a Branch router is using two ATM-IMA T1 links (3 Mbps combined throughput) to<br />
connect the Branch to the WAN, then the Branch switch could be configured to shape all WAN-destined<br />
traffic to 3 Mbps or could be configured to shape on a per-application basis to smaller increments.<br />
Refer to the queuing/dropping sections of these platforms in this chapter and Cisco IOS documentation<br />
for additional guidance on enabling shaping.<br />
Finally, if the combined WAN circuit-rate is significantly below 100 Mbps and the Catalyst switch<br />
does not support shaping, enable egress policing (when supported).<br />
If the Catalyst switch does not support shaping, then egress policing is the next-best alternative for this<br />
scenario.<br />
For example, the Catalyst 3550 does not support shaping, but it does support up to 8 policers on all egress<br />
ports. Thus it could still protect its Branch Router from being overwhelmed by policing on egress. Egress<br />
policing may be done on an aggregate level or on a per-application-basis.<br />
Again, the objective is to discard, as intelligently as possible, traffic that will inevitably be dropped<br />
anyway (by the WAN/Branch router) but, whenever possible, perform the dropping within Catalyst<br />
hardware (as opposed to IOS software).<br />
Egress policers are configured in the same manner as ingress policers, but the direction specified in the<br />
service-policy interface-configuration statement will be out, not in.<br />
Note The only Catalyst switch discussed in this chapter that did not support either shaping or egress policing<br />
is the Catalyst 2950. Unfortunately there is no way that the Catalyst 2950 can offload <strong>QoS</strong> from the<br />
Branch router. If such functionality is required, then a hardware upgrade would be advisable.<br />
Note For a case study example of Campus <strong>QoS</strong> design, refer to Figure 12-32 and Examples 12-76 through<br />
12-81 of the Cisco Press book, End-to-End <strong>QoS</strong> <strong>Network</strong> <strong>Design</strong> by Tim Szigeti and Christina Hattingh.<br />
<strong>Enterprise</strong> <strong>QoS</strong> <strong>Solution</strong> <strong>Reference</strong> <strong>Network</strong> <strong>Design</strong> <strong>Guide</strong><br />
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