Non-IP Header Marking Fields

·         When IP packet passing through an internetwork, the packet is encapsulated in a variety of other headers.

·         In these other headers have QoS fields that can be used for Classification & Marketing.

·         Ethernet LAN Class of Service

o   Ethernet supports a 3-bit QoS  marking field

§  This field only exist when the Ethernet header includes either an 802.1Q or ISL trunking header.

o   IEEE802.1Q defines its QoS field as the 3 most significant bits of the 2-byte Tag Control field, calling the field the Class of Service (CoS).

o   Regardless  of the type of trunking most people & most ios commands refer to these fields as CoS.

o   Bellow figure shows the general location of the CoS field inside ISl and 802.1P headers.

·         WAN Marking Fields

o   Frame Relay & ATM supports a single bit that can be set for QoS purpose,

§  But these single bits are intended for a very strict use related to drop probability.

o   Frames or cells with these bits sets to 1 are considered to be better candidates to be dropped than frames or cells without the bit set to 1.

o   These single bits are named as Frame Relay Discard eligibility (DE) bit and the ATM Cell Loss Priority (CLP) bit.

o   These bits can be sent by a router, or by an ATM or Frame Relay Switch.

o   Routers & Switches drop features can be configured more aggressively drop frames & cells that having the DE or CLP bit.

o   MPLS defines a 3-bit field called the MPLS Experimental (EXP) bit

§  It is used for general QoS marking.

§  Often, C&M tools are used on the edge of MPLS networks

·         to remap DSCP or IPP values to MPLS Experimental bit values

·         to provide QoS inside the MPLS network