a simple QoS technical question

Answered Question
Oct 18th, 2009
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According to my reading about catalyst switches, I understand one thing, that is whatever you trust, the system will eventually put change to DSCP for future QoS process. So, what is the main issue to configure truest CoS, or ip-precedence for?

Correct Answer by Peter Paluch about 7 years 6 months ago

Hi Julie,


I am not sure if I understand you correctly, but basically - if you have only IP traffic then having the trust configured for DSCP values should be the way to go. For non-IP traffic, the CoS markings should be trusted, however, these markings are preserved only on trunk links within a switched topology. If a 802.1p priority-tagged frame enters a switchport, the tag will be removed on the egress port until that port is a trunk port. The DSCP markings provide a much more "stable" place for priority markings, however, they are available only for IP.


I am not sure if I answered your question so please feel free to ask further.


Best regards,

Peter


Correct Answer by Peter Paluch about 7 years 6 months ago

Hello,


You can see the trust towards IP Precendence as a backward compatibility option for equipment that is not yet DSCP-aware or compatible. Admittedly, as the DSCP is here for a couple of years, the number of IP Precendence-based IP implementations is in my opinion diminishing.


Regarding the trust towards CoS marking in 802.1p/802.1Q, you have to consider the fact that not all Ethernet frames carry IP packets. There may be other types of payload, such as IPX, AppleTalk, CLNP and many others that do either do not have provisions for QoS markings or the switch does not support those. Also, some other low-end switches may understand only 802.1p/802.1Q priority markings and disregard DSCP markings.


Also note that if 802.1p/802.1Q markings and DSCP/IPP are both present, you choose what you want to trust and the other value will be rewritten according to a preset map so that both values represent the same priority as far as possible.


I am sure other colleagues here will extend my answer, as this is an issue I've been struggling myself with quite long, and I am looking forward to it.


Best regards,

Peter


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Correct Answer
Peter Paluch Sun, 10/18/2009 - 22:43
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  • Cisco Employee,

Hello,


You can see the trust towards IP Precendence as a backward compatibility option for equipment that is not yet DSCP-aware or compatible. Admittedly, as the DSCP is here for a couple of years, the number of IP Precendence-based IP implementations is in my opinion diminishing.


Regarding the trust towards CoS marking in 802.1p/802.1Q, you have to consider the fact that not all Ethernet frames carry IP packets. There may be other types of payload, such as IPX, AppleTalk, CLNP and many others that do either do not have provisions for QoS markings or the switch does not support those. Also, some other low-end switches may understand only 802.1p/802.1Q priority markings and disregard DSCP markings.


Also note that if 802.1p/802.1Q markings and DSCP/IPP are both present, you choose what you want to trust and the other value will be rewritten according to a preset map so that both values represent the same priority as far as possible.


I am sure other colleagues here will extend my answer, as this is an issue I've been struggling myself with quite long, and I am looking forward to it.


Best regards,

Peter


julxu Mon, 10/19/2009 - 16:07
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Great thank, Peter,


So, what a port of low end switch trust is not a problem, if the machine using ip it can configure as CoS trust, and if the it is ip packet it is DSCP trust, right?


So, I can put voice servers end for DSCP trust, and appletak PC end for CoS trust, and cisco equipment will map the CoS to DSCP, and QoS based on it, right?


Please confirm

Correct Answer
Peter Paluch Tue, 10/20/2009 - 00:37
User Badges:
  • Cisco Employee,

Hi Julie,


I am not sure if I understand you correctly, but basically - if you have only IP traffic then having the trust configured for DSCP values should be the way to go. For non-IP traffic, the CoS markings should be trusted, however, these markings are preserved only on trunk links within a switched topology. If a 802.1p priority-tagged frame enters a switchport, the tag will be removed on the egress port until that port is a trunk port. The DSCP markings provide a much more "stable" place for priority markings, however, they are available only for IP.


I am not sure if I answered your question so please feel free to ask further.


Best regards,

Peter


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