Hi folks. I'm glancing thru my BCMSN textbook as a way to in fact relax between BSCI cram sessions.
Cisco DiffServ QoS takes the standard IP ToS byte and formats it into a three-bit Class Selector value and a three-bit Drop Precendence value.
The chart I'm looking at compares these values with their counterparts in the old IP Precedence (non-DSCP) QoS model. If the chart is correct, there are 7 Class Selector values under IP Precedence, but only 4 under DSCP.
It's not superclear to me why they allocate three bits if there's only four possible values (all of which can fit into a two-bit space), but I'm assuming they're leaving the third bit for future growth.
But are there in fact only four Class Selector values? The text goes on to discuss Classes 5-7 without expressly saying that they exist only under IP precedence. Clarification would be most welcome.
Also, a diagram showing the format of the ToS/DS byte labels the final two bits as "ECN1" and "ECN0." What's that all about; what does ECN mean?