Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 68)

Chapter 2 Campus Infrastructure Considerations

Quality of Service

2-34

Cisco IP Telephony Network Design Guide

78-11103-03

Traffic Classification at Layer 3

Using the 802.1p bits within the 802.1Q tag provides the desired QoS results at

Layer 2. When traffic has to cross a Layer 3 boundary, however, it becomes

imperative to implement these mechanisms using Layer 3 parameters, such as the

3 IP precedence bits (commonly referred to as ToS) or the new DSCP parameter,

which uses the six most significant bits within the ToS byte of the IP header.

Traffic crosses a Layer 3 boundary when packets are routed between subnets by

Layer 3 switches or routers. Traffic also crosses a Layer 3 boundary when packets

need to go out of the campus network onto the WAN through edge routers. When

this happens, Layer 2 classification does not help. Layer 3 classification is needed

for achieving the desired level of QoS. All of the QoS techniques employed by the

routers (including the very important WAN QoS) rely on Layer 3 classification.

Layer 3 classification can be achieved by using the appropriate platforms in the

campus. Beginning with the IP phones, packets are already presented to the switch

with CoS = ToS = 5. This Layer 3 classification is preserved even if the packets

travel all the way through to the WAN edge router where the Layer 2 header is

removed. So, if the trust boundary is at the source (IP phone), voice traffic has the

ToS bits set to 5 and is presented to the network devices for appropriate treatment.

WAN routers can use this classification to employ any of the queuing techniques.

If the trust boundary is not at the source and packets need to be reclassified, then

the device performing this function should be capable of doing it at Layer 3 before

it can cross a Layer 3 boundary.

Layer 3 Traffic Classification on the Cisco Catalyst 6000

Cisco Catalyst 6000 family switches equipped with the Policy Feature Card (PFC)

perform Layer 3 traffic classification by default when the port is trusted. Thus if

a packet comes into a trusted port with CoS = 5, the switch takes this value and

resets the ToS bits to 5 as well. No additional configuration is needed. If the port

is untrusted, the packet gets a default CoS at the input port.

Then you can configure a QoS access control list (ACL) on the switch and rewrite

the ToS to a desired value based on some matching criteria. For example, the

following command sets a ToS of 5 for all packets coming from subnet 10.1.1.0

and destined to any address.

Console> (enable) set qos acl ip TEST dscp 40 10.1.1.0 0.0.0.255 any

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 68