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Understanding Aggregated Ethernet Interfaces

Link aggregation of Ethernet interfaces is defined in the IEEE 802.3ad standard. The Junos OS implementation of 802.3ad balances traffic across the member links within an aggregated Ethernet bundle based on Layer 3 information carried in the packet, Layer 4 information carried in the packet, or both, or based on session ID data. (The session ID data has higher precedence than the Layer 3 or 4 information.) This implementation uses the same load-balancing algorithm used for per-packet load balancing.

Aggregated Ethernet interfaces can be Layer 3 interfaces (VLAN-tagged or untagged) and Layer 2 interfaces.

Note: This topic is specific to the SRX3000 and SRX5000 line devices. For information about link aggregation for other SRX Series devices and J Series devices, see the Junos OS Layer 2 Bridging and Switching Configuration Guide for Security Devices.

This topic contains the following sections:

LAGs

You can combine multiple physical Ethernet ports to form a logical point-to-point link, known as a link aggregation group (LAG) or bundle, such that a media access control (MAC) client can treat the LAG as if it were a single link. Support for LAGs based on IEEE 802.3ad makes it possible to aggregate physical interface links on your device. LAGs provide increased interface bandwidth and link availability by linking physical ports and load-balancing traffic crossing the combined interface. For the LAG to operate correctly, it is necessary to coordinate the two end systems connected by the LAG, either manually or automatically.

Internally, a LAG is a virtual interface presented on SRX3000 and SRX5000 line devices or on any system (consisting of devices such as routers and switches) supporting 802.3ad link aggregation. Externally, a LAG corresponds to a bundle of physical Ethernet links connected between an SRX3000 or SRX5000 line device and another system capable of link aggregation. This bundle of physical links is a virtual link.

• The devices support a maximum of 16 physical interfaces per single aggregated Ethernet bundle.
• Aggregated Ethernet interfaces can use interfaces from the same or different Flexible PIC Concentrators (FPCs) and PICs.
• On the aggregated bundle, capabilities such as MAC accounting, VLAN rewrites, and VLAN queuing are available.

LACP

Junos OS supports the Link Aggregation Control Protocol (LACP), which is a subcomponent of IEEE 802.3ad. LACP provides additional functionality for LAGs, but is only supported on Layer 3.

LACP provides a standardized means for exchanging information between partner (remote or far-end of the link) systems on a link. This exchange allows their link aggregation control instances to reach agreement on the identity of the LAG to which the link belongs, and then to move the link to that LAG. This exchange also enables the transmission and reception processes for the link to function in an orderly manner.

For example, when LACP is not enabled, a local LAG might attempt to transmit packets to a remote individual interface, which causes the communication to fail. (An individual interface is a nonaggregatable interface.) 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.

You configure an aggregated Ethernet virtual link by specifying the link number as a physical device. Then you associate a set of ports that have the same speed and are in full-duplex mode. The physical ports can be 100-megabit Ethernet, 1-Gigabit Ethernet, and 10-Gigabit Ethernet.

• LACP does not support automatic configuration on SRX3000 and SRX5000 line devices, but partner systems are allowed to perform automatic configuration. When an SRX3000 or SRX5000 line device is connected to a fully 802.3ad-compliant partner system, static configuration of LAGs is initiated on the SRX3000 and SRX5000 line device side, and static configuration is not needed on the partner side.
• When an SRX3000 or SRX5000 line device is connected to a Juniper Networks MX Series router, static configuration of LAGs is needed at both the actor (local or near-end of the link) and partner systems.
• Although the LACP functions on the SRX3000 and SRX5000 line devices are similar to the LACP features on Juniper Networks MX Series routers, the following LACP features on MX Series routers are not supported on SRX3000 and SRX5000 line devices: link protection, system priority, and port priority for aggregated Ethernet interfaces. Instead, SRX3000 and SRX5000 line devices provide active/standby support with redundant Ethernet interface LAGs in chassis cluster deployments.

LACP is supported in standalone deployments, where aggregated Ethernet interfaces are supported, and in chassis cluster deployments, where aggregated Ethernet interfaces and redundant Ethernet interfaces are supported simultaneously.