The Type of Service (ToS) value B8 is used for the perhop behavior (PHB) value EF in the output of the show ip cache flow command. PHBs identify the Quality of Service (QoS) traffic class that is assigned to the packet. Expedited Forwarding (EF), which is defined in Request for Comments (RFC) 2598, indicates a high priority packet that should be given queuing priority over other packets but should not be allowed to completely monopolize the interface. Voice over IP (VoIP) traffic is often assigned a value of EF.

The TOS value in the output of the show ip cache flow command is an 8bit hexadecimal value. EF has a binary value of 10111000. To convert an eight digit binary value to a two character hexadecimal value, split the binary value in half and convert each section individually. The binary value 1011 converts to the hexadecimal character B, and the binary value 1000 converts to the hexadecimal character 8. Therefore, the value EF converts to the hexadecimal value B8, which is the TOS value that is displayed in the output of the show ip cache flow command.

Class Selector (CS) PHBs, which are defined in RFC 2475, only use the first three QoS bits; therefore, CS PHBs are backward compatible with 3bit IP precedence values. Packets with higher CS values are given queuing priority over packets with lower CS values.