Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?

Understanding Link Aggregation Control Protocol

 

LACP, a subcomponent of IEEE 802.3ad, provides additional functionality for link aggregation groups (LAGs). Use the link aggregation feature to aggregate one or more Ethernet interfaces to form a logical point-to-point link, known as a LAG, virtual link, or bundle. The MAC client can treat this virtual link like a single link.

Starting in Junos OS Release 15.1X49-D80, Link Aggregation Control Protocol (LACP) is supported in Layer 2 transparent mode in addition to existing support in Layer 3 mode for SRX300, SRX320, SRX340, SRX345, SRX1500, SRX4100, SRX4200 devices and vSRX instances. Starting in Junos OS Release 15.1X49-D40 and Junos OS Release 17.3R1, Link Aggregation Control Protocol (LACP) is supported in Layer 2 transparent mode in addition to existing support in Layer 3 mode for SRX5400, SRX5600 and SRX5800 devices. When the SRX Series device uses LACP to bundle the member links, it creates high-speed connections, also known as fat pipe, with peer systems. Bandwidth can be increased by adding member links. Increased bandwidth is important especially for redundant Ethernet (reth) and aggregated Ethernet (ae) interfaces, for transmitting and receiving packets to and from the peer end for the whole system. LACP also provides automatic determination, configuration, and monitoring member links. LACP is compatible with other peers that run the 802.3ad LACP protocol. It automatically binds the member links without manually configuring the LAG, thereby avoiding errors.

Note

Tentative sessions are created for all interfaces in a particular VLAN. If there is plenty of one-way traffic, numerous tentative sessions are created. When sessions reach the maximum limit, vector fails and packet loss might be seen.

This topic contains the following sections:

Link Aggregation Benefits

Link aggregation increases bandwidth, provides graceful degradation as failure occurs, and increases availability. It provides network redundancy by load-balancing traffic across all available links. If one of the links fails, the system automatically load-balances traffic across all remaining links.

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.

A typical LAG deployment includes aggregate trunk links between an access switch and a distribution switch or customer edge (CE) device.

Link Aggregation Configuration Guidelines

When configuring link aggregation, note the following guidelines and restrictions:

  • Link aggregation is supported only for Ethernet interfaces that are configured in switching mode (family ethernet-switching). Aggregating interfaces that are configured in routed mode (family inet) is also supported.

  • You can configure a LAG by specifying the link number as a physical device and then associating a set of ports with the link. All the ports must have the same speed and be in full-duplex mode. Junos OS assigns a unique ID and port priority to each port. The ID and priority are not configurable.

  • You can optionally configure LACP for link negotiation.

  • You can optionally configure LACP for link protection.

  • You can create up to eight Ethernet ports in each bundle. On QFX10016 device, you can create up to sixty four Ethernet ports in each bundle.

  • Each LAG must be configured on both sides of the link. The ports on either side of the link must be set to the same speed. At least one end of the LAG must be configured as active.

  • LAGs are not supported on virtual chassis port links.

  • 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 they receive them from another link. The transmitting link is known as the actor and the receiving link is known as the partner.

  • LAGs can only be used for a point-to-point connection.

For LACP configuration details, see Table 1 and Table 2.

Table 1: LACP (Link Aggregation Control Protocol) Configuration

Field

Function

Aggregated Interface

Indicates the name of the aggregated interface.

Link Status

Indicates whether the interface is linked (Up) or not linked (Down).

VLAN (VLAN ID)

Virtual LAN identifier value for IEEE 802.1Q VLAN tags (0.4094).

Description

The description for the LAG.

Table 2: Details of Aggregation

Field

Function

Administrative Status

Displays if the interface is enabled (Up) or disabled (Down).

Logical Interfaces

Shows the logical interface of the aggregated interface.

Member Interfaces

Member interfaces hold all the aggregated interfaces of the selected interfaces.

Port Mode

Specifies the mode of operation for the port: trunk or access.

Native VLAN (VLAN ID)

VLAN identifier to associate with untagged packets received on the interface.

IP Address/Subnet Mask

Specifies the address of the aggregated interfaces.

IPV6 Address/Subnet Mask

Specifies the IPV6 address of the aggregated interfaces.

For aggregated Ethernet interface options, see Table 3.

Table 3: Aggregated Ethernet Interface Options

Field

Function

Action

Aggregated Interface

Indicates the name of the aggregated interface.

Enter the aggregated interface name. If an aggregated interface already exists, then the field is displayed as read-only.

LACP Mode

Specifies the mode in which LACP packets are exchanged between the interfaces. The modes are:

  • None—Indicates that no mode is applicable.

  • Active—Indicates that the interface initiates transmission of LACP packets

  • Passive—Indicates that the interface only responds to LACP packets.

Select from the list.

Description

The description for the LAG.

Enter the description.

Interface

Indicates that the interfaces available for aggregation.

Click Add to select the interfaces.

Note: Only interfaces that are configured with the same speeds can be selected together for a LAG.

Speed

Indicates the speed of the interface.

Enable Log

Specifies whether to enable generation of log entries for LAG.

Select to enable log generation.

Note

On SRX100, SRX110, SRX210, SRX220, SRX240, SRX300, SRX320, SRX340, SRX345 and SRX650 devices, the speed mode and link mode configuration are available for member interfaces of ae. (Platform support depends on the Junos OS release in your installation.)

For VLAN options, see Table 4.

Table 4: Edit VLAN Options

Field

Function

Action

Port Mode

Specifies the mode of operation for the port: trunk or access.

If you select Trunk, you can:

  1. Click Add to add a VLAN member.

  2. Select the VLAN and click OK.

  3. (Optional) Associate a native VLAN ID with the port.

If you select Access, you can:

  1. Select the VLAN member to be associated with the port.

  2. (Optional) Associate a VoIP VLAN with the interface. Only a VLAN with a VLAN ID can be associated as a VoIP VLAN.

  3. Click OK.

VLAN Options

For trunk interfaces, the VLANs for which the interface can carry traffic.

Click Add to select VLAN members.

Native VLAN

VLAN identifier to associate with untagged packets received on the interface.

Select the VLAN identifier.

Release History Table
Release
Description
Starting in Junos OS Release 15.1X49-D80, Link Aggregation Control Protocol (LACP) is supported in Layer 2 transparent mode in addition to existing support in Layer 3 mode for SRX300, SRX320, SRX340, SRX345, SRX1500, SRX4100, SRX4200 devices and vSRX instances.
Starting in Junos OS Release 15.1X49-D40 and Junos OS Release 17.3R1, Link Aggregation Control Protocol (LACP) is supported in Layer 2 transparent mode in addition to existing support in Layer 3 mode for SRX5400, SRX5600 and SRX5800 devices.