ToS, or type of service markings are used primarily as L3 markings, whereas Cos, or Class of service is L2.
Here are the bits/meanings of them:
- Frame Relay Discard Eligible (DE) bit (a 1 or 0, 0 if not eligible for discard, 1 if it is)
- MPLS experimental bit
- Ethernet trunk CoS (3 bits)
- DSCP (differentiated services code point)
- IP precedence (older)
000 | 000 | 00
PHB (per-hop behavior) | drop reliability | flow control
Now DSCP is backwards-compatible with ip precedence because of those 3 left-most bits. Where DSCP differs itself is with the next 3 bits, the drop reliability. If you look back to that chart above you will see that decimal values 7 and 6 are reserved for network traffic, and that each 0-5 are assigned named values like assured or expedited forwarding. Now here higher is better; i.e. 101 or 5 is the best rating the packet can get according to the RFC. 000 being the worst. Drop probability on the other hand is opposite...higher is worse. Now drop reliability does NOT use all 3 of those bits...only the two left-most ones, so there are three possible combinations:
- 11x (High drop packet)
- 10x (Medium drop packet)
- 01x (Low drop)
AF13, you have assured forwarding 1, with a drop rating of high
AF41, you have assured forwarding 4, with a drop rating of low
If a router had to choose one to drop it would drop AF13 every time because it has a lower rating in both the IP precedence and drop rating.
They can also be viewed as decimal values:
10 = 001 010
So be careful when comparing two decimal values in terms of DSCP because higher or lower is not necessarily always better. Tomorrow night....dadada NBAR. Cant wait, see you then.