You are the network administrator of an enterprise with a main site and multiple remote sites. Your network carries both VOIP and data traffic. You agree with your service provider to classify VOIP and data traffic according to the different service RFCs. How can your data and VOIP traffic be marked?
A.
data marked with DSCP AF21, VOIP marked with DSCP EF
B.
data marked with DSCP AF51, VOIP marked with DSCP EF
C.
data marked with the DE-bit, VOIP marked with the CLP-bit
D.
data marked with DSCP EF, VOIP marked with DSCP AF31
E.
data marked with IP precedence 5, VOIP marked with DSCP EF
Explanation:
Expedited Forwarding
RFC 2598 defines the Expedited Forwarding (EF) PHB: "The EF PHB can be used to build a low loss, low latency, low jitter, assured bandwidth, end-to-end service through DS (Diffserv) domains. Such a service appears to the endpoints like a point-to- point connection or a "virtual leased line." This service has also been described as Premium service." Codepoint 101110 is recommended for the EF PHB, which corresponds to a DSCP value of 46.Again, vendor-specific mechanisms need to be configured to implement these PHBs. Refer to RFC 2598 for more information about EF PHB.
DSCP was designed to be more granular and more scalable than IP precedence BUT with backward compatibility.
The priority field (or type of service, ToS) was originally 3 bits, giving it the IP prec.values 0-7 (0 being the lowest priority, 7 the highest). DSCP has an 8 bit field, of which 6 bits are used for markings (the 6th bit is always 0). This gives it a larger number of values (both for per hop behavior, or PHB, and drop precedence. More on this later)
The last 2 bits are used for ECN or explicit congestion notification. This is a brand new feature (as of Windows Vista) and is purportedly going to revolutionize internetwork traffic flow. More on this later.
With the 6 bits allotted to DSCP, the first three (left to right) are used for Major Class, or Per Hop Behavior. These match up with the old IPprec values of 0-7.
The second 3 bits identify the drop precedence. Higher = more likely to be dropped. This means that a DSCP marking of AF21 (major class 2, drop precedence of 1) will be preferred over AF22 or AF23. An AF3x will beat any AF1x or AF2x–the major class of 3 is higher than the major class, or PHB, of 2.
It is important to note the drop precedence is only used on classes 1-4. (here is a table from wikipedia) A marking of 0 indicates ‘best effort’
Class 1 Class 2 Class 3 Class 4
Low Drop AF11 (DSCP 10) AF21 (DSCP 18) AF31 (DSCP 26) AF41 (DSCP 34)
Med Drop AF12 (DSCP 12) AF22 (DSCP 20) AF32 (DSCP 28) AF42 (DSCP 36)
High Drop AF13 (DSCP 14) AF23 (DSCP 22) AF33 (DSCP 30) AF43 (DSCP 38)The notation DSCP xx is the bit notation. Eg AF12 = 001 100 which is a decimal value of 12. Here’s another one. AF43 = 100 110 (38)
EF, which it equal to IPprec of 5 is assigned a major class of 5 and a drop pecedence of 3 (this is odd, I don’t know why this was done) The decimal value for EF is DSCP 46 or 101 110.