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Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
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Chapter 3 QoS in an AVVID-Enabled Campus Network
Selecting an Access-Layer Switch
Configuring the Uplink Interface to the Distribution Switch
The recommended design for wiring closet configurations of Catalyst 3500 XL family of Catalyst
switches is a star topology. A Catalyst 3500 family GigaStack configuration cannot provide guaranteed
voice QoS because it is essentially a shared media access model. The basic concern is the behavior of
the GigStack GBIC when you use both ports of the GBIC. GigaStack is a half-duplex connection.
Because it is half duplex, you cannot guarantee voice quality if you use both ports of the GigaStack
GBIC. So where you use stacked switches, you need to use only one port in the GigaStack GBIC. This
will mean that you use more GigaStack GBICS. However, this design will avoid any half- duplex,
non-deterministic behavior. Additionally, when you create stacks of switches you need to keep in mind
how your traffic will flow and how many devices will be participating in a Layer 2 convergence event.
In general, if you have to stack, design small stacks so that you will be able to tune spanning tree for fast
convergence.
Catalyst 3550 as an Access-Layer Switch
The Catalyst 3550 family of switches supports enhanced QoS features that can be used in the access
layer. Of particular interest are the Catalyst 3550's ability to classify and mark traffic on ingress to the
network using ACLs and policies. The Catalyst 3550's ability to identify traffic flows at Layer 3 and
Layer 4 using ACLs makes it very powerful as an access-layer device.
The Catalyst 3550 is a powerful, QoS-capable switch for the access layer. However, it does not support
the full set of features required for an IP telephony deployment. The Catalyst 3550 IOS software
provides full support for IP telephony. However, there is no hardware option that provides inline power.
This can be designed around through the use of the AC power brick or an inline power patch panel. The
inline power feature is scheduled for addition to the Catalyst 3550 family sometime in CY2002.
Figure 3-11
shows a general model for the Catalyst 3550 as an access device (as illustrated in the QoS
configurations discussed in this chapter).
Figure 3-11 General Model for Catalyst 3550 QoS Configurations
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Distribution
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