Understanding SRv6 Classification and Rewrite

In Segment Routing over IPv6 (SRv6) networks, classification and rewrite is crucial for managing complex networking scenarios. You can classify and rewrite SRv6 traffic at encapsulation, transit, and decapsulation nodes to enforce CoS marking consistency for L3VPN and EVPN traffic. Configure policies to map VLAN PCP and DSCP/DSCP-IPv6 code points and to rewrite VLAN tags and IPv6 headers. Policies support reduced and non-reduced modes, SID depths, micro or classic SID, dynamic tunnels, PSP or USP, and TILFA. You can enable DSCP propagation to copy access-side DSCP into the SRv6 IP header. DSCP propagation disables DSCP, DSCP-IPv6, and inet-precedence rewrite rules. You cannot classify traffic based on inner payload code points at decapsulation.

Benefits of SRv6 Classification and Rewrite

Enhances network performance by ensuring accurate handling of VLAN PCP bits and DSCP values, which is critical for maintaining QoS in complex networking scenarios such as L3VPN and EVPN.
Facilitates consistent DSCP value propagation across the network, ensuring uniform treatment of packets and maintaining service quality.
Supports a range of classification scenarios, allowing for precise traffic management and optimization tailored to specific network configurations and requirements.
Improves network adaptability and precision in SRv6 environments, offering advanced solutions to meet the demands of dynamic and scalable network infrastructures.

Overview

SRv6 classification and rewrite feature provides a comprehensive framework for managing packet attributes across SRv6 nodes, including encapsulation, transit, and decapsulation points. By implementing this feature, you can classify packets based on VLAN PCP bits and DSCP values. This classification is essential for maintaining optimal QoS in complex networking scenarios like L3VPN and EVPN, where precise traffic management ensures that network performance is not compromised. The encapsulation process uses these attributes to define how packets should be treated as they enter the SRv6 network.

A core component of this functionality is the ability to propagate DSCP values from native IP packets to the SRv6 header. This ensures DSCP settings are consistently applied across the network, maintaining uniform treatment of packets. However, enabling DSCP propagation affects certain rewrite rules, which you must consider during configuration to avoid unexpected behavior. By understanding these interactions, you can configure SRv6 nodes to apply classification and rewriting rules effectively, optimizing traffic management according to your specific network requirements.

The feature outlines various classification scenarios that help you configure the network more efficiently. These scenarios specify how different SRv6 nodes handle the VLAN PCP and DSCP bits, guiding you in setting up the system to achieve desired traffic outcomes. While the feature supports many classification and rewrite scenarios, it does not support classification based on inner payload code points at decapsulation nodes. This limitation requires careful planning to ensure configurations are error-free and network operations are reliable.

Classification and Rewrite at SRv6 Encapsulation Node

Classification and rewrite at an SRv6 encapsulation node support the following scenarios:

Table 1: Classification and Rewrite at SRv6 Encapsulation Node
Scenario	Classification	Rewrite
Reduced mode	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface
Non-reduced mode	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface
Segment Identifier more than 4	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface
Segment Identifier less than 4	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface
Best effort	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface
Dynamic tunnel	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface
L3VPN over SRv6	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface Preserve dscp/dscp-ipv6 bits in original packet (payload)
EVPN over SRv6 (ELAN, VPWS)	Classification based on vlan pcp bits on access side interface Classification based on dscp-ipv6 bits on access side interface Classification based on dscp bits on access side interface	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag on core facing interface Rewrite of dscp-ipv6 bits in SRv6 IPv6 header on core facing interface Preserve dscp/dscp-ipv6 bits in original packet (payload)

Classification and Rewrite at SRv6 Transit Node

Classification and rewrite at an SRv6 transit node support the following scenarios:

Table 2: Classification and Rewrite at SRv6 Transit Node
Scenario	Classification	Rewrite
Packet with one or more segment identifier (SID)	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header
Penultimate Segment Pop (PSP) when segment identifiers are less than 5	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header
Penultimate Segment Pop (PSP) when segment identifiers are more than 5	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header
Ultimate Segment Pop (USP)	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header
Reduced mode	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header
Binding SID	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header
TILFA non-insert mode	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header
TILFA insert mode	Classification based on vlan pcp bits in SRv6 packet outer vlan tag Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in SRv6 packet outer vlan tag Rewrite of dscp-ipv6 bits in SRv6 IPv6 header

Classification and Rewrite at SRv6 Decapsulation Node

Classification and rewrite at an SRv6 decapsulation node support the following scenarios:

Table 3: Classification and Rewrite at SRv6 Decapsulation Node
Scenario	Classification	Rewrite
Decapsulation with segment routing header	Classification based on vlan pcp bits in SRv6 packet Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in original packet for EVPN-SRv6. Preserve payload DSCP. Rewrite dscp-ipv6 bits in original packet in L3VPN-SRv6. In EVPN-SRv6, preserve vlan pcp bits in original packet if vlan pcp rewrite is not configured on outgoing interface.
Decapsulation without segment routing header	Classification based on vlan pcp bits in SRv6 packet Classification based on dscp-ipv6 bits in SRv6 IPv6 header	Rewrite of vlan pcp bits in original packet for EVPN-SRv6. Preserve payload DSCP. Rewrite dscp-ipv6 bits in original packet in L3VPN-SRv6. In EVPN-SRv6, preserve vlan pcp bits in original packet if vlan pcp rewrite is not configured on outgoing interface. In L3VPN-SRv6, preserve dscp-ipv6 bits in original packet if dscp-ipv6 rewrite not configured on outgoing interface
L3VPN over SRv6	Classification based on vlan pcp bits in SRv6 packet on core interface Classification based on dscp-ipv6 bits in SRv6 IPv6 header on core interface	Rewrite dscp-ipv6 bits in original packet on access interface In L3VPN-SRv6, Preserve dscp-ipv6 bits in original packet if dscp-ipv6 rewrite not configured on access interface
EVPN over SRv6 (ELAN, VPWS)	Classification based on vlan pcp bits in SRv6 packet on core interface Classification based on dscp-ipv6 bits in SRv6 IPv6 header on core interface	Rewrite of vlan pcp bits in original packet on access interface In EVPN-SRv6, Preserve vlan pcp bits in original packet if vlan pcp rewrite not configured on access interface