QoS question for IP Telephony

Answered Question

Hi,

I woul like understand some concept for my BCMSN certification exam about QoS for IP telephony.

1# CoS value is a layer 2 information add in layer 2 header of IP packet, I would like to understand why it's better to trust CoS value came from IP Phone than the DSCP value came from same device. Layer 2 information will be lost when the layer encapsulation will be removed and DSCP value will not….. What's append when when the packet need to be routed, i twill lost it's QoS information and this packet will be forward as any other data packet after router ? It's does makes sense to me….

2# What is difference between IP precedence and DSCP value and which one is better QoS classification method ?

thanks a lot for your help !!

I have this problem too.
0 votes
Correct Answer by Joseph W. Doherty about 8 years 9 months ago

Thank you.

Note how the recommendation isn't to IP phones in general, but to Cisco IP Phones and their inherent capabilities. Further notice the recommendation about setting CoS is because the Cisco IP Phones also behave as a mini-switch, and in this case, "Generally, for Cisco IP Phones, you should use the CoS keyword because the phone can control the CoS values on its two-VLAN trunk with the switch."

This appears to agree with my assumption, it seems a Cisco IP Phone, in its roles as the edge switch, can force CoS to be what is desired, but as a L2 device it doesn't seem to have the same capabilities of setting DSCP. The recommendation of using CoS vs. DSCP might not or need not apply to different vendor IP Phones.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Joseph W. Doherty Sun, 02/24/2008 - 13:21

2) IP precedence is based on an older RFC for usage of an IP packet's ToS byte. The newer RFC DSCP's interpretation is arguable better because it also allows for different drop precedences within the same class.

1) I'm curious where it was suggested it's better to trust CoS vs. ToS. Only reason I can think of for such an assumption, since CoS might usually be lost across a L3 hop, if it's still present, the network intends the value. Where with ToS, it might remain, by default, unchanged end-to-end and the network hasn't approved it.

Correct Answer
Joseph W. Doherty Mon, 02/25/2008 - 18:22

Thank you.

Note how the recommendation isn't to IP phones in general, but to Cisco IP Phones and their inherent capabilities. Further notice the recommendation about setting CoS is because the Cisco IP Phones also behave as a mini-switch, and in this case, "Generally, for Cisco IP Phones, you should use the CoS keyword because the phone can control the CoS values on its two-VLAN trunk with the switch."

This appears to agree with my assumption, it seems a Cisco IP Phone, in its roles as the edge switch, can force CoS to be what is desired, but as a L2 device it doesn't seem to have the same capabilities of setting DSCP. The recommendation of using CoS vs. DSCP might not or need not apply to different vendor IP Phones.

Actions

This Discussion