Understanding IEEE 802.3ad Link Aggregation

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 a 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.


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 an Ethernet link to actively or passively transmit PDUs by sending out LACP PDUs only when it receives the PDUs 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

Interface Stack for 802.3ad Link Aggregation

For information about configuring higher-level protocols over VLANs, see VLAN Overview.

Note: On the ES2 10G LM and ES2-S1 GE-8 IOA combination, you can configure only 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, which 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 Configuring Layer 2 Services over MPLS in JunosE BGP and MPLS Configuration Guide for more information.

IPv6 Packets and LAG

You can configure IPv6 prefix addresses for a LAG bundle and for VLAN and PPPoE subinterfaces that are members of a LAG bundle. In environments in which an independent or a dual-stack IPv6 subscriber exists, the PPP link between the customer premises equipment (CPE) and the provider edge (PE) router might require both IPv4 and IPv6 addresses for transmission of data. In such networks, you can use LAG bundles configured with IPv6 addresses for effective usage of bandwidth and reduced administrative costs.

The selection of the member interface in a LAG bundle is performed on the egress side of the module. For IPv6 traffic, the hash value is calculated using the IPv6 source and destination addresses. One of the links, based on the hashing of the IPv6 source and destination addresses, is used to send the packets out of the router. Hashed mode is the default equal-cost multipath (ECMP) mode of operation.

The egress side of the module contains eight segments. Each segment maps to one interface in the LAG bundle. If the member interfaces in a LAG bundle are fewer than eight, the segment is filled by repeating the member interface for that bundle.

You cannot aggregate links with IPv6 traffic from multiple line modules, bridge IPv6 packets over a LAG bundle, or use Link Aggregation Control Protocol (LACP) to dynamically configure LAG bundles with IPv6 addresses.

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