HP 6120G/XG HP ProCurve Series 6120 Blade Switches Advanced Traffic Management - Page 177
Terminology, Differentiated Services Codepoint., codepoint
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Quality of Service (QoS): Managing Bandwidth More Effectively Introduction Terminology Term Use in This Document 802.1p priority A traffic priority setting carried by a VLAN-tagged packet moving from one device to another through ports that are tagged members of the VLAN to which the packet belongs. This setting can be from 0 7. The switch handles an outbound packet on the basis of its 802.1p priority. However, if the packet leaves the switch through a VLAN on which the port is an untagged member, this priority is dropped, and the packet arrives at the next, downstream device without an 802.1p priority assignment. 802.1Q field A four-byte field that is present in the header of Ethernet packets entering or leaving the switch through a port that is a tagged member of a VLAN. This field includes an 802.1p priority setting, a VLAN tag, or ID number (VID), and other data. A packet entering or leaving the switch through a port that is an untagged member of the outbound VLAN does not have this field in its header and thus does not carry a VID or an 802.1p priority. See also "802.1p priority". codepoint Refer to DSCP, below. downstream device A device linked directly or indirectly to an outbound switch port. That is, the switch sends traffic to downstream devices. DSCP Differentiated Services Codepoint. (Also termed codepoint.) A DSCP is comprised of the upper six bits of the ToS (Type-of-Service) byte in IP packets. There are 64 possible codepoints. In the default QoS configuration for the switches covered in this guide, some codepoints are configured with default 802.1p priority settings for Assured-Forwarding and Expedited Forwarding. All other codepoints are unused (and listed with No-override for a priority). DSCP policy A DSCP configured with a specific 802.1p priority (0- 7). (Default: No-override). Using a DSCP policy, you can configure the switch to assign priority to IP packets. That is, for an IP packet identified by the specified classifier, you can assign a new DSCP and an 802.1p priority (0-7). For more on DSCP, refer to "Details of QoS IP Type-of-Service" on page 5-24. For the DSCP map, see figure 5-10 on page 5-25. edge switch In the QoS context, this is a switch that receives traffic from the edge of the LAN or from outside the LAN and forwards it to devices within the LAN. Typically, an edge switch is used with QoS to recognize packets based on classifiers such as TCP/UDP application type, IP-device (address), Protocol (LAN), VLAN-ID (VID), and Source-Port (although it can also be used to recognize packets on the basis of ToS bits). Using this packet recognition, the edge switch can be used to set 802.1p priorities or DSCP policies that downstream devices will honor. inbound port Any port on the switch through which traffic enters the switch. IP Options In an IPv4 packet, optional, these are extra fields in the packet header. IP-precedence The upper three bits in the Type of Service (ToS) field of an IP packet. bits IPv4 Version 4 of the IP protocol. outbound packet A packet leaving the switch through any LAN port. outbound port Any port on the switch through which traffic leaves the switch. 5-6