When a service is bound to an SRv6 tunnel, the service packets are first forwarded to the egress network interface of the SRv6 origination FPE to build and push the SRv6 encapsulation. The packets are then handed in to the ingress network interface of the SRv6 origination FPE which sprays the packets over the ECMP next hops of the SRv6 tunnel and LAG links of the outgoing network interfaces.
The default hash calculation on the ingress service SAP or interface is based on the existing hash procedures of an IPv4, IPv6, or an Ethernet packet. For IPv6 service packets, an option is provided to include the packet’s Flow Label field, when not zero, and to hash on the triplet {SA, DA, Flow Label}. The flow-label-load-balancing command is used to enable this behavior on an access or network interface.
The SRv6 origination FPE egress network interface copies the output of the hash on the inner packet headers into the flow label field of the outer IPv6 header that it pushes on the SRv6 encapsulated packet. This is regardless of whether the flow label is used or not in the computation of the hash on the service packet.
The SRv6 origination FPE ingress network interface does not require the flow-label-load-balancing command to be enabled. All SRv6 packets are automatically sprayed to the ECMP next hops of the SRv6 tunnel and LAG links of the outgoing network interfaces using a hash on the triplet {SA, DA, Flow Label} in the SRv6 packet’s outer IPv6 header.
On a transit router, the hashing of SRv6 encapsulated packets can also use the Flow Label field in the outer IPv6 header to provide more entropy to the load-balancing process of SRv6 packets. The flow-label-load-balancing command can be configured on a network interface to hash on the triplet {SA, DA, Flow Label}. By default, a transit router only hashes on the tuple {SA, DA} in the header of a received IPv6 packet with a non-zero flow label field, including when the packet is SRv6. The description of the flow-label-load-balancing command and the detailed behavior of the hash feature based on the IPv6 packet flow label field and its general application to access and network interfaces is described in section IPv6 Flow Label Load Balancing of the 7450 ESS, 7750 SR, 7950 XRS, and VSR Interface Configuration Guide .