Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 63)

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Cisco IP Telephony Network Design Guide

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Chapter 2 Campus Infrastructure Considerations

Quality of Service

Trust Boundaries

The concept of trust is an important and integral one to implementing QoS. Once

the end devices have a set class of service (CoS) or type of service (ToS), the

switch has the option of trusting them or not. If the switch trusts the settings, it

does not need to do any reclassification; if it does not trust the settings, then it

must perform reclassification for appropriate QoS.

The notion of trusting or not trusting forms the basis for the trust boundary.

Ideally, classification should be done as close to the source as possible. If the end

device is capable of performing this function, then the trust boundary for the

network is at the access layer in the wiring closet. If the device is not capable of

performing this function, or the wiring closet switch does not trust the

classification done by the end device, the trust boundary may shift. How this shift

happens, depends on the capabilities of the switch in the wiring closet. If the

switch can reclassify the packets, then the trust boundary remains in the wiring

closet. If the switch cannot perform this function, then the task falls to other

devices in the network going toward the backbone. In this case, the rule of thumb

is to perform reclassification at the distribution layer. This means that the trust

boundary has shifted to the distribution layer. It is more than likely that there is a

high-end switch in the distribution layer with features to support this function. If

possible, try to avoid performing this function in the core of the network.

In summary, try to maintain the trust boundary in the wiring closet. If necessary,

move it down to the distribution layer on a case-by-case basis, but avoid moving

it down to the core of the network. This advice conforms with the general

guidelines to keep the trust boundary as close to the source as possible.

Note This discussion assumes a three-tier network model, which has

proven to be a scalable architecture. If the network is small, and the

logical functions of the distribution layer and core layer happen to

be in the same device, then the trust boundary can reside in the core

layer if it has to move from the wiring closet.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 63