350-001 Cisco Certified Internetworking Expert Study Guide

Demo Question 12.

Answer: E

Explanation: The Per-Hop Behavior is indicated by encoding a 6-bit value-called the Differentiated Services Code Point (DSCP)-into the 8-bit Differentiated Services (DS) field of the IP packet header. In theory, a network could have up to 64 (26) different traffic classes using different markings in the DSCP. The DiffServ RFCs recommend, but do not require, certain encodings, which gives a network operator great flexibility in defining traffic classes. In practice, however, most networks use the following commonly-defined Per-Hop Behaviors: Default PHB. which is typically best-effort traffic Expedited Forwarding (EF. PHB. for low-loss, low-latency traffic Assured Forwarding (AF. -behavior group Class Selector PHBs-which are defined to maintain backward compatibility with the IP Precedence field. Prior to DiffServ, IP networks could use the Precedence field in the Type of Service (TOS) byte of the IP header to mark priority traffic. The TOS byte and IP precedence was not widely used. The IETF agreed to reuse the TOS byte as the DS field for DiffServ networks. In order to maintain backward compatibility with network devices that still use the Precedence field, DiffServ defines the Class Selector PHB. The Class Selector codepoints are of the form 'xxx000'. The first three bits are the IP precedence bits. Each IP precedence value can be mapped into a DiffServ class. If a packet is received from a non-DiffServ aware router that used IP precedence markings, the DiffServ router can still understand the encoding as a Class Selector codepoint. Reference: http://en.wikipedia.org/wiki/Differentiated_services