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Understanding Aggregated Ethernet Interfaces and LACP

Link Aggregation Group (LAG)

You configure a LAG by specifying the link number as a physical device and then associating a set of interfaces (ports) with the link. All the interfaces must have the same speed and be in full-duplex mode. Juniper Networks Junos operating system (Junos OS) for EX Series Ethernet Switches assigns a unique ID and port priority to each interface. The ID and priority are not configurable.

The number of interfaces that can be grouped into a LAG and the total number of LAGs supported on a switch varies according to switch model. Table 1 lists the EX Series switches and the maximum number of interfaces per LAG and the maximum number of LAGs they support. MX Series devices can support up to 64 LAGs.

When configuring LAGs, consider the following guidelines:

• You must configure the LAG on both sides of the link.
• You must set the interfaces on either side of the link to the same speed.
• You can configure and apply firewall filters on a LAG.
• You can optionally configure LACP for link negotiation.
• You can optionally configure LACP for link protection.

You can combine physical Ethernet ports belonging to different member switches of a Virtual Chassis configuration to form a LAG. See Understanding EX Series Virtual Chassis Port Link Aggregation and Understanding Link Aggregation into an EX8200 Virtual Chassis.

Note: The interfaces that are included within a bundle or LAG are sometimes referred to as member interfaces. Do not confuse this term with member switches, which refers to switches that are interconnected as a Virtual Chassis. It is possible to create a LAG that is composed of member interfaces that are located in different member switches of a Virtual Chassis.

A LAG creates a single logical point-to-point connection. A typical deployment for a LAG would be to aggregate trunk links between an access switch and a distribution switch or customer edge (CE) router.

Link Aggregation Control Protocol (LACP)

When LACP is configured, it detects misconfigurations on the local end or the remote end of the link. Thus, LACP can help to prevent communication failure:

• When LACP is not enabled, a local LAG might attempt to transmit packets to a remote single interface, which causes the communication to fail.
• When LACP is enabled, a local LAG cannot transmit packets unless a LAG with LACP is also configured on the remote end of the link.

By default, Ethernet links do not exchange protocol data units (PDUs), which contain information about the state of the link. You can configure Ethernet links to actively transmit PDUs, or you can configure the links to passively transmit them, sending out LACP PDUs only when the Ethernet links receive the PDUs from another link. The transmitting link is known as the actor and the receiving link is known as the partner.

In a scenario where 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 might 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 such a scenario, 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.