This documents explains the configuration knobs 'flow-label-transmit-static' and 'flow-label-receive-static' and how they are used and configured on Juniper devices

Syntax:

flow-label-transmit-static;

flow-label-receive-static;

Hierarchy Level:

[edit protocols l2circuit neighbor neighbor-id interface interface-name],

[edit routing-instances instance-name protocols evpn],

[edit routing-instances instance-name protocols evpn interface interface-name]

A flow label is a 28-bit value optionally added to the header of an IP packet. It can be used for various purposes, including traffic engineering and Quality of Service (QoS) differentiation.

In the context of L2VPNs and L2 Circuits , a flow label is optionally added to the header of an MPLS (Multiprotocol Label Switching) packet carrying L2 traffic. It plays a specific role in load balancing data flows across multiple paths within the MPLS network. 

Flow-aware transport of pseudowires (FAT) flow labels enable load balancing of MPLS packets across equal-cost multipath (ECMP) paths or link aggregation groups (LAGs) without the need for deep packet inspection of the payload. 

You can use this statement to enable devices to push FAT flow labels for:

Forwarding equivalence class (FEC) 128 pseudowires in:

• Virtual private LAN service (VPLS) networks.
• Virtual private wire service (VPWS) networks.

In these cases, configure this statement in the [edit protocols l2circuit neighbor neighbor-id interface interface-name] hierarchy.

Flow-label (transmit/receive) config can be configured dynamically or statically under pseudowire configuration as per the requirement.

These knobs configure static flow labels, they are used to statically configure a specific flow label value for pseudowire packets transmitted or received by a Juniper router interface.

flow-label-transmit-static:

• Sets a static flow label value for PW packets outgoing from the router interface to the remote PE device
• The router inserts/pushes this pre-defined flow label into the header of all PW packets it transmits.

flow-label-receive-static:

• Sets a static flow label value for PW packets incoming to the router interface.
• The router expects the incoming PW packets to have the pre-defined flow label value which is popped once received from remote PE device. If the flow label doesn't match, the router might drop the packet (depending on configuration).

Configuration on PE device:

[edit protocols l2circuit]
user@PE1# show
neighbor a.b.c.d {
    interface ge-1/0/x.0 {
        virtual-circuit-id 1;
        flow-label-transmit-static;
        flow-label-receive-static;
    }
}

Note :

Ideally to ensure optimal interoperability and load balancing, it's generally recommended to match the configuration on the local and remote PE device's in regards to usage of this knob.

Please refer the following Juniper technical documentation for more details on static flow-label configuration,

https://www.juniper.net/documentation/us/en/software/junos/vpn-l2/topics/task/vpns-flow-labels-fec-128-vpws-pw-configuring.html