CoS and DSCP Changes for a Voice Packet
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Trust
boundary if
the IP Phone
is trusted.
Trust
boundary if
the IP Phone
is untrusted,
but the Access
switch is trusted.
Trust
boundary if
the Access
switch is
untrusted, but
Distribution
switch is trusted.
Trust
boundary if
ISP if the
customer’s
markings are
not trusted.
ISP Network
Phone sets CoS to 5 and
DSCP to 46 on voice
traffic, before forwarding
to Access switch.
Switch is configured to trust
the IP phone. It leaves the
DSCP value uncharged,
and recreates the CoS of 5
when forwarding to
Distribution switch.
Data DSCP = 46
IP Header
CoS = 5
802.1p bits
Data DSCP = 46
IP Header
CoS = 5
802.1p bits
Data DSCP = 46
IP Header
Switch is configured to trust
Access switch. It give QoS
service to packet when
sending to router, does not
change DSCP.
Router is configured to trust
Distribution switch. It gives
QoS service to packet where
sending to WAN, does not
change DSCP.
Data DSCP = 46
IP Header
Data DSCP = 46
IP Header
ISP does trust customer
router, leaves DSCP
unchanged at 46, sets MPLS
EXP to 5. MPLS network
provides QoS service
contracted for the packet.
■ The Cisco IP phone creates an 802.1Q trunk between itself and the
Access switch. It sets the 802.1p CoS on its own voice media traffic to
5, and it sets the Layer 3 marking to EF or DSCP 46 (binary value
101110). If this were voice signaling traffic, the phone would set a
CoS of 3 and a Layer 3 marking of CS3, or DSCP 24 (binary value
011000).
■ The Access switch gets the frame from the phone and strips the Layer
2 header. It is configured to trust the IP phone, so it translates the CoS
value into an internal DSCP as the packet moves through the switch
fabric. The switch then translates that internal DSCP value back to a
CoS of 5 in the 802.1Q tag when it sends the packet to the Distribution
switch. The Access switch applies any outbound policies configured
for this traffic, such as putting it into a priority interface queue, as it
sends it to the outbound interface.
■ The Distribution switch is configured to trust the Access switch’s
markings. It strips off the Layer 2 header, looks at the DSCP value,
and provides the type of QoS service it is configured to provide to EF
traffic. This typically includes placing the packet in a priority queue at
the interface. The switch’s interface to the router is a Layer 3 interface,
so no trunk tag is used. Instead it puts on a normal Ethernet frame
header and forwards the packet to the router.
■ The router is configured to trust the packet’s markings. It strips off the
Layer 2 header, looks at the DSCP value, and provides the type of QoS
service it is configured to provide to EF traffic. This typically includes
placing the packet in an LLQ and allocating a certain amount of bandwidth
to that queue. The router then forwards the packet to its ISP
edge router.
■ The ISP is configured to trust the customer’s markings. It translates the
IP precedence value of 5 into an MPLS Experimental Bits value of 5.
The DSCP stays unchanged. The packet receives prioritized, real-time
service as it moves through the ISPs network, and devices in the destination
network can use the unchanged DSCP values to also provide
QoS service to the packet.
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