Classifying and Marking in a VoIP Network
Figure 4-2 shows what happens to the CoS and DSCP settings of a data
packet as it moves through a QoS-enabled LAN.
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CoS and DSCP Changes for a Data Packet
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Trust
boundary if
the PC is
trusted
(rarely done).
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
the Distribution
switch is trusted.
Trust
boundary if
ISP if the
customer’s
markings are
not trusted.
ISP Network
PC with 802.1Q-capable NIC
sends a packet through the
phone. PC user sets the
packet’s DSCP to 56 (IP
precedence 7) and CoS to 7.
Data DSCP = 56
IP Header
CoS = 7
802.1p bits
Phone sets CoS on PC
traffic to 0 before
forwarding to Access
switch. Does not change
the DSCP value.
Data DSCP = 56
IP Header
CoS = 0
802.1p bits
Switch is configured classify
and mark data traffic. It sets
both DSCP and CoS on PC
traffic to 1 before forwarding
to Distribution switch.
Data DSCP = 1
IP Header
CoS = 1
802.1p bits
Data DSCP = 1
IP Header
Switch is configured to trust
Access switch. It gives 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 whnen
sending to WAN, does not
change DSCP.
Data DSCP = 1
IP Header
Data DSCP = 1
IP Header
ISP does not trust customer
router, leaves DSCP at 1 but
sets MPLS EXP to 0. Traffic
will be given best-effort QoS
through provider’s network.
■ In the figure, users with an 802.1Q-enabled Network Interface Card
(NIC) on their PC attempts to give their data higher priority within the
network. They send a frame with an 802.1Q tag in which they have set
the 802.1p bits to CoS of 7. They have also set the DSCP on the packet
to 56. This animation shows just the relevant parts of the headers used.
■ The IP phone by default creates an 802.1Q trunk between itself and the
Access switch. It sets the 802.1p CoS on data traffic to zero, but it
does not change any Layer 3 markings.
■ The Access switch gets the frame from the phone and strips the Layer
2 header. By default it translates into an internal DSCP of zero as it
moves through the switch fabric; however, this switch is configured to
classify and mark data traffic. This particular application falls into a
class that gets a Layer 3 marking of AF11, or DSCP 10 (binary value
001010). The switch remarks the DSCP value, and then sets the CoS to
1 in the 802.1Q tag when it sends the packet to the Distribution switch.
■ 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
AF11 traffic. 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 AF11 traffic. This might include
allocating a certain amount of bandwidth and using Weighted Random
Early Detection (WRED) in the queue. The router then forwards the
packet to its ISP edge router.
■ The ISP is not configured to trust the customer’s markings. It could
overwrite all DSCP values with zero, but in this case it just sets the
MPLS Experimental Bits in the MPLS label to zero. The DSCP stays
unchanged. The packet receives only best-effort service as it moves
through the ISPs network, but devices in the destination network can
use the unchanged DSCP values to provide QoS service to the packet.
Figure 4-3 shows what happens to the CoS and DSCP settings of a voice
packet as it moves through a QoS-enabled LAN. In this example, the ISP
trusts the customer’s markings.