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    Understanding Aggregated Ethernet Interfaces and LACP

    IEEE 802.3ad link aggregation enables you to group Ethernet interfaces to form a single, aggregated Ethernet interface, also known as a link aggregation group (LAG) or bundle.

    Link aggregation is used to aggregate Ethernet interfaces between two devices. You can create a LAG between a Juniper Networks device and a router, switch, aggregation switch, server, or other devices. The aggregated Ethernet interfaces that participate in a LAG are called member links. Because a LAG is composed of multiple member links, even if one member link fails, the LAG continues to carry traffic over the remaining links.

    Link Aggregation Contol Protocol (LACP) is a subcomponent of the IEEE 802.3ad standard and is used as a discovery protocol.

    Note: To ensure load balancing across the aggregated Ethernet (AE) interfaces on a redundant server Node group, the members of the AE must be equally distributed across the redundant server Node group.

    Note: During a network Node group switchover, traffic might be dropped for a few seconds.

    Link Aggregation Group

    To create a LAG:

    1. Create a logical aggregated Ethernet interface.
    2. Define the parameters associated with the logical aggregated Ethernet interface, such as a logical unit, interface properties, and Link Aggregation Control Protocol (LACP).
    3. Define the member links to be contained within the aggregated Ethernet interface—for example, two 10-Gigabit Ethernet interfaces.
    4. Configure LACP for link detection.

    Keep in mind these hardware and software guidelines:

    • Up to 64 Ethernet interfaces can be grouped to form a LAG, and up to 448 LAGs are supported on QFX3500, QFX3600, QFX5100, and EX4600, and OCX Series switches.

      Note: If you try to commit a configuration containing more than 64 Ethernet interfaces in a LAG, you will receive an error message saying that the group limit of 64 has been exceeded, and the configuration checkout has failed.

    • The LAG must be configured on both sides of the link.
    • The interfaces on either side of the link must be set to the same speed and be in full-duplex mode.

      Note: On a QFX5100 and EX4600 standalone switch or QFX5100 Virtual Chassis and EX4600 Virtual Chassis, you can configure mixed rate aggregated Ethernet bundles (LAGs with different link speeds).

      Note: Junos OS assigns a unique ID and port priority to each port. The ID and priority are not configurable.

    Link Aggregation Control Protocol (LACP)

    LACP is one method of bundling several physical interfaces to form one logical aggregated Ethernet interface. The LACP mode can be active or passive. The transmitting link is known as the actor, and the receiving link is known as the partner. If the actor and partner are both in passive mode, they do not exchange LACP packets, and the aggregated Ethernet links do not come up. If either the actor or partner is active, they do exchange LACP packets. By default, LACP is in passive mode on aggregated Ethernet interfaces. To initiate transmission of LACP packets and response to LACP packets, you must enable LACP active mode. You can configure Ethernet links to actively transmit protocol data units (PDUs), or you can configure the links to passively transmit them, sending out LACP PDUs only when they receive them from another link. You can configure both VLAN-tagged and untagged aggregated Ethernet interfaces without LACP enabled. LACP is defined in IEEE 802.3ad, Aggregation of Multiple Link Segments.

    LACP was designed to achieve the following:

    • Automatic addition and deletion of individual links to the LAG without user intervention.
    • Link monitoring to check whether both ends of the bundle are connected to the correct group.

    When a dual-homed server is deployed with a switch, the network interface cards form a LAG with the switch. During a server upgrade, the server may not be able to exchange LACP PDUs. In such a situation you can configure an interface to be in the up state even if no PDUs are exchanged. Use the force-up statement to configure an interface when the peer has limited LACP capability. The interface selects the associated LAG by default, whether the switch and peer are both in active or passive mode. When there are no received PDUs, the partner is considered to be working in the passive mode. Therefore, LACP PDU transmissions are controlled by the transmitting link.

    If the remote end of the LAG link is a security device, LACP might not be supported because security devices require a deterministic configuration. In this case, do not configure LACP. All links in the LAG are permanently operational unless the switch detects a link failure within the Ethernet physical layer or data link layers.

    Modified: 2017-03-14