QOS for switch and router

Say i have a voip phone connected to a swich and i use auto qos on the switch for all teh FA and trunk port to the router.  On the router do i need auto qos trust on both the ehternet connection to the switch and the serial interface outbound or just the serial interface ourtbound?

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maxis2cuteAsked:
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kyleb84Commented:
Both, because there are always 2 streams when using VoIP, one stream to the VoIP device, and a stream to the destination. Therefore you want the router to obey QoS values at each side of the router (Fa + Ser)
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maxis2cuteAuthor Commented:
i dont disagree, i just want to be 100 percent.  If i have auto qos trunst on the switchport trunk interface, does that not trust the markings from the router?  and if i dont have trust on the ethernet connection to the router will it drop the markings or will it reclassify?

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kyleb84Commented:
trust qos will only trust ingress (from device, into a port) traffic, outgoing traffic (from the router, to a device) is automatically trusted.

If you have no "trust qos" on your switchport interface all outgoing traffic is trusted, incoming is reclassified.

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maxis2cuteAuthor Commented:
so one more time and then i will accept your answer.  

switch QOS trust on a voice port--trust the cos from phone
switch QOS trust on trunk port-trust ip prec from router
qos trust on ethernet--trust markings from switch w/o reclassifying, if no trust will it reclasify or drop marking?

qos serial service policy output will send to next router.

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kyleb84Commented:
"switch QOS trust on a voice port--trust the cos from phone"
- Bingo

switch QOS trust on trunk port-trust ip prec from router
- (Yes) by default, that command will trust DSCP from the router.

qos trust on ethernet--trust markings from switch w/o reclassifying, if no trust will it reclasify or drop marking?
- Yes, and...
- (no trust) Reclassify if QoS policy is applied to router/switch, remove QoS if no policy.

qos serial service policy output will send to next router.
- The next router won't receive service-policy details of the previous, only COS/DSCP values.
- You must explicitly set the ip prec in a global policy applied to that interface.
See attached code.

!
! 'match' can be anything that will catch your VoIP traffic.
!
class-map voicetraffic
 match protocol rtp audio
!
policy-map defaultqos
 class voicetraffic
  set ip precedence 4
!
interface Serial 0/0/0
 service-policy output defaultqos
!

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