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# 802.3ad Link Aggregation for Ethernet Overview

IEEE 802.3ad link aggregation enables you to group Ethernet interfaces at the physical layer to form a single link layer interface, also known as a link aggregation group (LAG) or bundle. For more information, see IEEE Standard 802.3ad, Link Aggregation.

Some users require more bandwidth in their network than a single Fast Ethernet link can provide, but cannot afford the expense or do not need the bandwidth of a higher-speed Gigabit Ethernet link. Using IEEE 802.3ad link aggregation in this situation provides increased port density and bandwidth at lower cost. For example, if you need 450 Mbps of bandwidth to transmit data and have only a 100-Mbps Fast Ethernet link, creating a LAG bundle containing five 100-Mbps Fast Ethernet links is more cost effective than purchasing a single Gigabit Ethernet link.

For information about the modules that support link aggregation, see ERX Module Guide, Appendix A, Module Protocol Support and E120 and E320 Module Guide, Appendix A, IOA Protocol Support.

## LACP

The Link Aggregation Control Protocol (LACP) is a mechanism for exchanging port and system information to create and maintain LAG bundles. The LAG bundle distributes MAC clients across the link layer interface and collects traffic from the links to present to the MAC clients of the LAG bundle.

To create the links in the LAG bundles, you can add one or more Ethernet physical interfaces to it. The LACP detects Ethernet interfaces as links if they are configured on the same line module and have the same physical layer characteristics. The LACP also assigns to the LAG bundle the same MAC address of the Ethernet link with the highest port priority, which is the lowest value.

The LACP also controls the exchange of LACP protocol data units (PDUs) between the Ethernet links in the LAG bundle. The PDUs contain information about each link and enable the LAG bundle to maintain them.

By default, Ethernet links do not exchange PDUs, which contain information about the state of the link. You can configure Ethernet links to actively transmit PDUs, or passively transmit them, sending out LACP PDUs only when it receives them from another link. The transmitting link is known as the Actor and the receiving link is known as the Partner.

## Higher-Level Protocols

After you configure the LAG bundle, you can route IP traffic over it, create a VLAN over it, route PPPoE traffic over it, or route MPLS traffic over it.

Figure 20 displays the interface stack for 802.3ad link aggregation.

Figure 20: Interface Stack for 802.3ad Link Aggregation

For information about configuring higher-level protocols over VLANs, see Configuring VLAN and S-VLAN Subinterfaces .

| Note: On the ES2 10G LM and ES2-S1 GE-8 IOA combination, you can only configure IP or VLAN over a LAG bundle.

## Load Balancing and QoS

You can configure load balancing across 802.3ad links to provide quality of service (QoS). To ensure that QoS is symmetrically applied to all the links, the router periodically rebalances the traffic on the LAG. When you attach a QoS profile to the LAG, the load balancing properties that are configured are applied to the LAG, and determines how traffic is distributed.

For example, if VLANs are configured, IP queues are provisioned over the VLANs. In this case, the default behavior is per-VLAN load balancing.

For more information, see Providing QoS for Ethernet Overview.

## Ethernet Link Aggregation and MPLS

CE-side load balancing in a Martini layer 2 transport environment enables an E-series router to interoperate with an 802.3ad switch in a topology designed for Ethernet link aggregation. See chapter Configuring Layer 2 Services over MPLS in JUNOSe BGP and MPLS Configuration Guide for more information.

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