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I often come across in networking related literature the term 'traffic class'. And it looks like it is platform specific, sometimes they talk about 8 traffic classes, sometimes about 64.

1) Is this actually determined by underlying ASIC?

2) Does it have something to do with CoS value encoded in VLAN tag?

Thanks.

2 Answers 2

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Your question is too abstract.

  • it depends. If you are referring to the number of hardware queues, then yes, this is determined by the underlying hardware (like in Cisco, Juniper). But queues can be handled by the software, then their number is determined by some sort of [hard]coded limits (like in Linux/tc/netfilter, FreeBSD/ipfw/pf). But basically you can have a large number of traffic classes. The only limitation is that you will be probably unable to assing all of them to a limited number of hardware queues, if you have these in hardware.

  • it can. In fact, you are asking about classifiers. A QoS-enabled router can classify traffic using several methods: by packet IP header, by the interface the traffic goes in, by the ToS header of the IP datagram, by the TCP/UDP header, by the protocol type, by the DSCP field of the 802.1Q header (that's what you asked about), or it can combine all of these. Classifying packets using DSCP gives you an advantage of marking and remarking packets, so they would be processed similary on a set of connected similary configured QoS-enabled routers, this way you can control how traffic goes from the traffic source - it can be handy.

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The old school QOS (802.1p) there were 0-7 qos classes. Diffserv allows up to a theoretical 64 values

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