IPv6 flow label load balancing enables load balancing in ECMP and LAG based on the output of a hash performed on the triplet {SA, DA, Flow-Label} in the header of an IPv6 packet received on a IES, VPRN, R-VPLS, CsC, or network interface.
IPv6 flow label load balancing complies with the behavior described in RFC 6437. When the flow-label-load-balancing command is enabled on an interface, the router applies a hash on the triplet {SA, DA, Flow-Label} to IPv6 packets received with a non-zero value in the flow label.
If the flow label field value is zero, the router performs the hash on the packet header; using the existing behavior based on the global or interface-level commands.
When enabled, IPv6 flow label load balancing also applies hashing on the triplet {SA, DA, Flow-Label} of the outer IPv6 header of an SRv6 encapsulated packet that is received on a network interface of a SRv6 transit router.
At the ingress PE router, SRv6 supports inserting the output of the hash that is performed on the inner IPv4, IPv6, or Ethernet service packet header into the flow label field of the outer IPv6 header it pushes on the SRv6 encapsulated packet.
For more details of the hashing and spraying of packets in SRv6, see the 7750 SR and 7950 XRS Segment Routing and PCE User Guide.
The flow label field in the outer header of a received IPv6 or SRv6 encapsulated packet is never modified in the datapath.