Combining TOS and IP Precedence to Mimic DSCP
You can additionally get the agnate of DSCP, alike on earlier routers that abutment alone TOS and Precedence, by accumulation the TOS and Precedence values. All Assured Forwarding DSCP Class 1 ethics are agnate to an IP Precedence amount of 1, Priority. All Class 2 ethics accord to IP Precedence 2, Immediate; Class 3 ethics to IP Precedence 3, Flash; and Class 4 corresponds to an IP Precedence amount of 4, Flash Override. The college IP Precedence ethics are not acclimated for Assured Forwarding.
You can again baddest the adapted Bead Precedence accumulation from the TOS values. However, you accept to be a little bit careful, back there are 4 TOS bits. Accumulation this with the 3 $.25 from IP Precedence gives you 7 $.25 to appointment with, while DSCP alone uses the aboriginal 6. For example, attractive at the bit ethics that accord AF11 in Table B.3, you can see that the aftermost three $.25 are 010. So the agnate TOS amount would be 0100, which is 4 in decimal, or best throughput.
In Table B.3, you can see that a TOS amount of 4, best throughput, consistently gives the everyman Assured Forwarding Bead Precedence. Selecting a TOS amount of 8, minimum delay, gives Medium Bead Precedence in all classes. And you can get the accomplished Assured Forwarding Bead Precedence amount by ambience a TOS amount of 12, which doesn't accept a accepted name in the TOS terminology.
There is analytic acceptable interoperability amid the Assured Forwarding DSCP variables and the aggregate of IP Precedence and TOS, which is acceptable because it is absurd for a router to acquaint the aberration in general. Alone the three top priorities of IP Precedence are not represented, and that is artlessly because these DSCP ethics are acclimated for affirmed commitment services.
The better aberration amid the TOS and Assured Forwarding models is that area the Assured Forwarding archetypal is acclimated to ascertain a blazon of queueing, the TOS archetypal is acclimated to baddest a accurate path. TOS was advised to appointment was with a acquisition protocol, such as OSPF, in selecting the best adapted aisle for a accurate packet based on its TOS value. That is, back there are assorted paths available, a TOS-based OSPF (such as the adaptation appropriate in RFC 2676) would attack to accomplish a reasonable TOS appointment to anniversary of them. Then, if the router bare to advanced a packet that was apparent with a accurate TOS value, it would attack to acquisition a avenue with the aforementioned TOS value. Note that Cisco never congenital this blazon of functionality into its OSPF implementation, however. So, if TOS is activity to accept an aftereffect on how packets are forwarded, you accept to configure it manually by agency of Policy-based routing.
This was the actual absorbed for TOS, but in convenance best engineers begin that it was easier to aloof use the TOS acreage to access queueing behavior rather than aisle selection. So the IETF developed the added avant-garde and advantageous DSCP formalism.
Assured Forwarding introduces the abstraction of Per-Hop Behavior (PHB). Anniversary DSCP amount has a agnate categorical PHB that the router uses not to baddest a path, but to ascertain how it will advanced the packet. The router will advanced a packet apparent AF13 forth the aforementioned arrangement aisle as a packet apparent AF41 if they both accept the aforementioned destination address. However, it will be added acceptable to bead the AF13 packet if there is congestion, and it will advanced the AF41 packet aboriginal if there are several packets in the queue.
From this, it should be bright why it is easier to apparatus Assured Forwarding than TOS-based acquisition on a network.