Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?

 
ON THIS PAGE
 

DSCP Propagation and Default CoS on ACX Series Routers

Junos OS Evolved uses default CoS and rewrite functions on ACX Series routers. As a result, Differentiated Services code points (DSCP) can't propagate from the native payload to the VXLAN tunnel header by default.

Note:

Use the statement set system packet-forwarding-options no-ip-tos-rewrite to enable DSCP propagation for VXLAN traffic. When you configure or delete DSCP propagation, the evo-pfemand process restarts.

This statement can handle IP code point propagation, but VLAN Priority Code Point (PCP) propagation is not supported.

Benefits of DSCP Propagation and Default CoS Support for EVPN VXLANs

The set system packet-forwarding-options no-ip-tos-rewrite statement enables the DSCP field from the incoming packet to propagate to the VXLAN encapsulated packet. This allows CoS to be applied to VXLAN encapsulated packets as they traverse the network.

Default CoS and DSCP propagation on VXLAN interfaces

By default, the DSCP information in the incoming (native) payload does not propagate to the VXLAN tunnel header. Therefore, the DSCP of the VXLAN packet is set to zero. With the set system packet-forwarding-options no-ip-tos-rewrite statement, the DSCP of the native payload propagates to the VXLAN tunnel header.

The Layer 3 classifier (inet-precedence, DSCP, DSCPv6) at the ingress interface classifies packets to a forwarding class and loss priority. With the set system packet-forwarding-options no-ip-tos-rewrite statement, the Layer 3 classifier classifies packets using the DSCP of the native packet since the DSCP of the native payload is always retained.

The Table 1 provide a summary of the CoS, Layer 3, and L3 and L2 classifier behavior when you enable the set system packet-forwarding-options no-ip-tos-rewrite statement.

Table 1: CoS Behavior with CLI Statement

CoS Behavior with CLI Statement

CoS Behavior without CLI Statement

Disables L3 rewrite

Existing CoS function

Retains the DSCP of the native payload

Retains the DSCP of the native payload

Enables DSCP propagation for both VXLAN L2 and VXLAN L3, with the L3 classifier explicitly attached to the user-to-network (UNI) interface for an L2.

DSCP of the VXLAN packet is set to zero

Layer 3 Classifier Behavior

Table 2: Layer 3 Classifier Behavior

Ingress Classifier Type

Egress Rewrite Type

Ingress Packet DSCP

Support for Rewrite as per Configured Rewrite

L3 (DSCP, DSCP IPv6, IP Precedence) on L2 or L3 IFL

L3

not applicable

Preserves only the configured rewrite

L3 (DSCP, DSCP IPv6, IP Precedence) on L2 or L3 IFL

IEEE 802.1p/ad, EXP

not applicable

Preserves only the configured rewrite

Does not allow rewrite or preserve the configured rewrite for IEEE 802.1p and IEEE 802.1pad/EXP

L2 (IEEE 802.1p/ad) on L2 IFL

IEEE 802.1p/ad

not applicable

Supports rewrite and preserves if DSCP is present

MPLS EXP

IEEE 802.1p/ad, MPLS EXP

not applicable

Yes

L3 (DSCP, DSCP IPv6, IP Precedence)

IEEE 802.1p/ad, EXP

0 to 7

The rewrite configuration is preserved if the ingress DSCP matches the forwarding class id used in the rewrite rule. For IEEE 802.1p, EXP rewrite occurs per the rewrite rule IEEE 802.1p, EXP rewrite occurs as per the rewrite rule (forwarding class, loss priority to IEEE 802.1p/EXP).

For IEEE 802.1p, the EXP bits are set to zero.

L3 (DSCP, DSCP IPv6, IP Precedence)

IEEE 802.1p/ad, EXP

More than 7

For IEEE 802.1p, the EXP bits are set to zero

The Table 3 provides a summary of the L3 and L2 classifier behavior when you enable the CLI statement.

Table 3: L3 and L2 Classifier Behavior with CLI Statement

Input Packet

Type of Interface at UNI

Classifier

User Configuration

Ingress Remark

Egress Rewrite

VXLAN Encapsulated Packet DSCP

Description

L3 IPv4 and IPv6

IRB/L3 UNI

DSCP default

None

No

No

PCP/DEI of inner packet

None

L2 carrying IPv4 and IPv6

L2

PCP default

DSCP classifier

No

No

PCP/DEI of inner packet

User needs to configure DSCP classifier explicitly on L2 UNI.

Pure L2 (no IP payload)

L2

PCP default

DSCP classifier

No

No

PCP /DEI of inner packet having VLAN tag

Example: PCP/DEI bits = 100/1 => DSCP = 1001 =9 (decimal); DSCP =0 with untagged packet; DSCP=0 with native tagging

 User needs to configure DSCP classifier explicitly on L2 UNI; On QFX, DSCP=0 on VXLAN encapsulated packet irrespective of single tagged, untagged and native tagged packets at L2 UNI.

Limitations

When you enable the set system packet-forwarding-options no-ip-tos-rewrite statement on an ACX Series router:

  • The IP DSCP or IP Precedence rewrite is disabled in the system and network. As a result, you can't execute L3 rewrite (DSCP, DSCP IPv6, IP precedence).

  • IEEE 802.1p code point bits in the packet header retain their original header. IEEE 802.1p propagation is not supported.

Related Documentation