Assured Forwarding Model De-Mystified

Now that we have an understanding of how the individual bit fields of the DSCP are factored, we will look at the entire assured forwarding model and examine the intention behind the model. If all six bits of the DSCP field are off (zero), the decimal value is also zero. While a marking of zero is normally referred to as best effort (BE), the assured forwarding model uses the keyword of default in reference to this particular marking (the lack of marking). If the first three bits of the DSCP field begin with 001, the packet belongs to assured forwarding class 1. The next two bits need to be examined to extract the drop preference of the packet. If the next two bit fields are both zero (00), the packet is marked as cs1 (class selector 1). Recall from previous blogs, that the class selectors exist to allow backward compatibility with the older IP Precedence QoS marking model. If the drop preference bits are not both off (0), there is a drop preference configured for the class. The four possibilities for class 1 are as follows: Keyword Binary Decimal CS1 001 00 0 8 AF11 001 01 0 10 AF12 001 10 0 12 AF13 001 11 0 14 AF11 indicates the packet belongs to assured forwarding (AF) class 1 and the drop preference is 1 (low). AF12 indicates the packet belongs to AF class 1, drop preference of 2 (medium). AF13 indicates the packet belongs to AF class 1, drop preference 3 (high). High drop preference traffic is normally dropped (discarded) before medium drop preference traffic. Low drop preference traffic is dropped last. Since CS1 does not have a drop preference, it is normally set to the same drop preference as CS1. The drop preferences of DSCP markings are very closely aligned with congestion avoidance mechanisms that we will cover in future blogs. Weighted Random Early Detection (WRED) is the predominant congestion avoidance mechanism deployed on Cisco routers. We will cover the distinctions of this model and talk about Cisco’s Weighted Tail Drop (WTD) mechanism used in some of Cisco’s newer switch platforms (3560, 3750, 2960, 2970). Notice that a higher decimal value does not indicate higher priority in this model. The keyword and binary values are the keys in unlocking the meaning of the assured forwarding model. The entire assured forwarding model is as follows: Keyword Binary Decimal CS1 001 00 0 8 AF11 001 01 0 10 AF12 001 10 0 12 AF13 001 11 0 14 CS2 010 00 0 16 AF21 010 01 0 18 AF22 010 10 0 20 AF23 010 11 0 22 CS3 011 00 0 24 AF31 011 01 0 26 AF32 011 10 0 28 AF33 011 11 0 30 CS4 100 00 0 32 AF31 100 01 0 34 AF32 100 10 0 36 AF33 100 11 0 38 Notice that class selector 1 has a decimal value of 8 and subsequent class selectors are quantities of 8 (8, 16, 24, and 32). Within each class, the decimal values go up by two. The mathematics can be factored using shortcuts like this, or the old fashion way. Line up the bit values to the binary decimal number system. 32 16 8 4 2 1 1 0 0 0 1 0 A A A D D 0 AF1 DP1 AF11 We will continue our discussion in the next blog…