BA and PHB

BA and PHB Actions

The QoS actions take by the device always depends on the service-class and color of the packet.

The service-class and color of a packet is initialized to <BE, Green>.
If the trust upstream command is configured on the inbound interface, when received a packet, the upstream (inbound) board of the device resets the service-class and color of the packet based on the priority field(s) (such as DSCP, 802.1p, EXP, and so on) of the packet. This procedure is called BA action.
If remark, or remark within CAR, is configured for the packet, the inbound board resets the service-class and color of the packet.
Then, the device takes other QoS actions based on the service-class and color of the packet.
After the above actions are taken, the downstream (outbound) board should decide whether to modify the priority field(s) (such as DSCP, 802.1p, EXP, and so on) of the packet or not. In some scenarios, the priority field of the packet is not expected to be changed. The procedure that the outbound board modifies the priority field(s) of the packet based on the service-class and color of the packet is called PHB action.
Figure 1 BA and PHB actions

BA and PHB Symbols

A device sets two markers on each interface to determine whether to implement reverse mapping for outgoing packets.

Marker 1: BA. BA is enabled on the inbound interface and is carried in internally added packet headers and transmitted to the outbound interface through the switching module.
Marker 2: PHB. PHB is enabled on the outbound interface.

The device takes PHB action only when both the two symbols are set as "Y".

By default, the "BA-symbol" is set as "N", and the "PHB-symbol" is set as "N".

Both the "BA-symbol" and "PHB-symbol" can be changed by commands (Table 1).

**Table 1** Commands for setting the "BA-symbol" and "PHB-symbol"
Command	QoS Action	Values of the "BA-symbol" and "PHB-symbol"
trust upstream	The board takes BA action for inbound packets and takes PHB action for outbound packets.	Both the "BA-symbol" and "PHB-symbol" are set as "Y".
diffserv-mode { pipe \| short-pipe }	The board takes BA action for inbound packets.	The "BA-symbol" and "PHB-symbol" keeps unchanged.
diffserv-mode uniform	This command is default configuration and does not affect the actions of the inbound and outbound boards.	Both the "BA-symbol" and "PHB-symbol" keep unchanged.
service-class	The service-class and color of the packet are reset.	The "BA-symbol" is set as "Y" if the parameter no-remark is not configured within the command. The "BA-symbol" is set as "N" if the parameter no-remark is configured within the command.
remark (inbound)	The "BA-symbol" is set as "Y" and the "PHB-symbol" is not affected. The inbound board takes BA action and remarks the inbound packets, regardless of the "BA-symbol" and "PHB-symbol".
remark (outbound)	The outbound board remarks the outbound packets, regardless of the "BA-symbol" and "PHB-symbol". For example, assume that the remark dscp 11 command is configured for outbound interface and the service-class and color of the packet are <ef, green>. The DSCP of the packet is set as 11 directly, rather than the value mapped from <ef, green> based on the downstream PHB mapping table. If the outbound packet has vlan tag, the 802.1p of the vlan tag is set based on <ef, green> and the downstream PHB mapping table. If both the remark dscp 11 command and the remark 8021p command are configured for the outbound interface, then both the DSCP and the 802.1p of the packet are modified directly according to the remark commands.
qos phb enable	-	The "PHB-symbol" is set as "Y".
qos phb disable	-	The "PHB-symbol" is set as "N".
qos car { green \| yellow \| red } pass service-class color	The service-class and color of the packet are reset.	Both the "BA-symbol" and "PHB-symbol" keep unchanged.

Rules for PHB Action

To set BA-symbol as "Y", configure the "trust upstream", "remark", "service-class" command on inbound interface.
To set BA-symbol as "N", configure the "service-class class-value color color-value no-remark" command, or do not configure the four commands stated above on the inbound interface.
To set PHB-symbol as "Y", configure the "trust upstream" or "qos phb enable" command on outbound interface.
To set PHB-symbol as "N", configure the "qos phb disable" command or do not configure the "trust upstream" command on outbound interface.

**Table 2** Rules for PHB action
BA-symbol	PHB-symbol	PHB Action
N	N	No
Y	N	No
Y	Y	Yes

Which QoS Priority Field is Trusted If a Packet Carries the DSCP, 802.1p, and MPLS EXP Fields

The trusted QoS priority fields depend on the configuration on the inbound interface. For details, see Table 3.

**Table 3** Trusted priority fields
Inbound Interface Configuration		Packet Type	Which Field Is Trusted
trust upstream	trust 8021p	Packet Type	Which Field Is Trusted
×	×	Any types	No field is trusted and the packet is mapped to <BE, Green>.
×	√	Any types
√	×	IPoE, IPoVLAN, IPoQinQ	DSCP
		IPoPPP, IPoHDLC, IPoFR	DSCP
		MPLS	Outer EXP (if EXP, 802.1p, and DSCP are all available, EXP is also selected)
		Other types	No field is trusted. The recognized packets are mapped to <CS6, Green>. Usually, these recognized packets are protocol packets, such as IS-IS packets, PPPoE packets, and ARP packets. So they are mapped to <CS6, Green>. Unrecognized packets are mapped to <BE, Green>. Usually, the unrecognized packets are error packets, so they are mapped to <BE, Green>.
√	√	VLAN	802.1p
		QinQ	802.1p in outer VLAN
		MPLS	If EXP, 802.1p, and DSCP are all available, select 802.1p.
		Others	No field is trusted and the packet is mapped to <BE, Green>.
NOTE: "Other types" here indicates neither IP nor MPLS packets when the outer L2 header is removed.

Which Priority Field of the Inbound Packet Is Reset in PHB Action

**Table 4** Which Priority Field Is Reset in PHB Action
Outbound Interface Configuration		Priority Field Re-marked in PHB Action
trust upstream	trust 8021p	Priority Field Re-marked in PHB Action
Not configured	Not configured	No field is re-marked.
Not configured	Configured	No field is re-marked.
Configured	Not configured	MPLS packet: Outer EXP Non-MPLS packet: DSCP
Configured	Configured	MPLS packet: 802.1p and EXP Others: Outer 802.1p

Rules for Marking the 802.1p Field of New-added VLAN Tag

When a VLAN header is added to a packet after the packet passes through a device, the setting of the 802.1p value depends on the PHB-symbol status.

**Table 5** Rules for Marking the 802.1p Field of New-added VLAN Tag
PHB-symbol	Rules for marking the 802.1p field of the new-added VLAN tag
Y	According to the <service-class, color> of the packet and the downstream priority mapping table.
N	Marked as 0.

Rules for Marking the EXP Field of the New-added MPLS Header

When an MPLS header is added to a packet after the packet passes through a device, the setting of the EXP value depends on the PHB-symbol status.

**Table 6** Rules for marking the EXP field of the new-added MPLS header
PHB-symbol	Rules for Marking the EXP Field of the New-added MPLS Header
Y	Both inner and outer EXP: according to the <service-class, color> of the packet and the downstream priority mapping table.
N	Outer EXP: according to the service-class of the packet: If service-class=BE, then Exp=0. If service-class=AF1, then Exp=1. If service-class=AF2, then Exp=2. If service-class=AF3, then Exp=3. If service-class=AF4, then Exp=4. If service-class=EF, then Exp=5. If service-class=CS6, then Exp=6. If service-class=CS7, then Exp=7. Inner EXP of VPLS: the same as the mark method of the outer EXP. Inner EXP of L3VPN or VLL: according to the <service-class, color> of the packet and the PHB mapping table of the default DS domain on the downstream outbound interface.