Can someone explain CoS?

Answered Question
Nov 4th, 2008

When does CoS come into play? How many values are there, and in what priority (low number is low priority or is higher number?)


Where can I find more information on CoS values? I've looked, and it seems like I can only find references to them in MPLS docs.


Thanks!

John

Correct Answer by Jon Marshall about 8 years 3 months ago

John


"So is IP precedence (0-7) the L3 equivalent to CoS"


Kind of. There are the same number of values and so you could map them across. And because DSCP is backwards compatible with IP Prec then you can map CoS to DSCP which is exactly what a switch does.


IP precedence is a subset of DSCP as DSCP used more bits in the ToS byte than IP precedence.


Jon

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Jon Marshall Tue, 11/04/2008 - 10:17

John


You could also search on 802.1p as this is CoS marking.


There are 8 CoS values ranging from 0 which is best effort to 7 which is actually reserved as is 6. 5 is voice.


CoS markings are used at layer 2 in ethernet frames and are present within an 802.1q packet. This bit is quite important to understand ie. you need an 802.1q vlan tagged packet to be able to have CoS markings as they are contained within the 802.1q header. So you will see CoS markings present on a trunk link.


CoS is purely a way to mark and prioritise packets at layer 2 on ethernet.


Jon



John Blakley Tue, 11/04/2008 - 10:24

Thanks Jon.


So is IP precedence (0-7) the L3 equivalent to CoS? Is the IP precedence also DSCP?


Thanks,


John

Correct Answer
Jon Marshall Tue, 11/04/2008 - 10:32

John


"So is IP precedence (0-7) the L3 equivalent to CoS"


Kind of. There are the same number of values and so you could map them across. And because DSCP is backwards compatible with IP Prec then you can map CoS to DSCP which is exactly what a switch does.


IP precedence is a subset of DSCP as DSCP used more bits in the ToS byte than IP precedence.


Jon

John Blakley Tue, 11/04/2008 - 10:40

Thanks Jon :-) I'm trying to get a handle on this whole classification and marking thing with QoS.



cisco_lad2004 Tue, 11/04/2008 - 10:52

One thing to keep in mind is the Remapping as Jon mentioned.


You could take an IP packet mark it with DSCP40 for example, packet is then trunked and frame marked with COS2...when Frame comes out of teh switch and Vlan tag is removed ...so does the COS mapping and ur packet arrives at destination unaffected by COS marking.


Luckily for us...most cisco switches have internal tables mapping COS-DSCP or COS-IPREC.


Morality, marking can be useless if not applied end to end.


HTH


Sam

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