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Configuring Control Word for EVPN-MPLS


Transit devices in an EVPN-MPLS network are not aware of the type of payload it carries. While parsing an MPLS encapsulated packet for hashing, a transit device can incorrectly calculate an Ethernet payload as an IPv4 or IPv6 payload if the first nibble of the destination address MAC is 0x4 or 0x6, respectively, causing out-of-order packet delivery. To identify the payload as an Ethernet payload, we can insert a control word with a value of 0 in the first four bits between the label stack and the L2 header of the packet. This ensures that the packet is not identified as an IPv4 or IPv6 packet.


You do not need to enable control word on the devices in your transit network when the transit devices consists of Juniper Networks EX 9200 switches, MX series routers, or PTX Series routers. These Juniper devices correctly identify the Ethernet payload as an IPv4/IPv6 payloads, even when the Ethernet destination MAC address starts with 0x4 or 0x6 nibble. The Juniper devices perform hashing based on the IP header fields inside the Ethernet frame and will not send out-of-order packets. In this case, we recommend not using control word as there are no benefits.

To enable control word, set control-word for the evpn protocol for a specified routing instance.

The following output shows a sample multihomed routing instance with control word configured.

user@router1# show routing-instances

To view routes where control word is supported, use the show route table mpls.0 protocol evpn operational command. Egress routes display an offset of 252.

show route table mpls.0 protocol evpn