admin@Xorplus# set interface aggregate-ethernet ae1 aggregated-ether-options lacp fallback timeout 0
admin@Xorplus# commit

admin@Xorplus# set protocols spanning-tree enable false
admin@Xorplus# commit

admin@Xorplus# set protocols spanning-tree mstp interface ae1 manual-forwarding true
admin@Xorplus# commit

Overview

The LACP fallback feature sets the LACP LAG port to fallback mode and brings up one of the member ports to the active (up) state before any LACP PDUs is received by the LAG port. The fallback mode will not exit until one LACP PDU is received by any one of the member ports from the peer device.

Application Scenarios

This feature is useful for scenarios where the peer device is a Preboot Execution Environment (PXE) booting client and connected through a LAG interface on Pica8 switch. With LACP fallback feature enabled on Pica8 switch, a PXE booting client can complete the PXE booting process and system configurations by connecting to the DHCP server/TFTP server through one of the active LAG member ports on Pica8 switch.

The following sections describe in detail the application scenarios of LACP Fallback in both non-MLAG and MLAG networks.

Non-MLAG Networks

As shown in Figure 1, in Non-MLAG networks, where the peer device connected to the Pica8 Switch is a Server or ToR through an LACP LAG port on Pica8 Switch.

Such Server and ToR are PXE booting clients, which cannot send and process LACP messages during the PXE phase. This causes the connected LAG port on Pica8 Switch to be inactive and unable to communicate with the PXE booting clients. These clients need the link to be active so that they can reach the DHCP or TFTP server to get the necessary installation files.

Figure 1. LACP Fallback Applied in Non-MLAG Networks

With LACP fallback feature, a Pica8 Switch will allow the system to bring up the LAG (before receiving any LACP PDUs from the PXE booting clients) and keeps one of the member ports active until receiving an LACP PDU by one of the member ports from the peer. This allows the PXE booting clients to establish a connection, download its boot image and then continue the boot process. When the boot process is complete, the server will fully support to form an LACP LAG.

MLAG Networks

As shown in Figure 2, in MLAG networks or an AmpCon application scenario, LACP Fallback can be configured on the spine switches. The newly deployed Leaf switch/ToR/Server in bare metal state with no NOS installed, will not be able to send and process LACP messages during the PXE phase. This causes the LAG port connected on the Spine switches to be inactive and hence making it impossible for the Leaf switch/ToR/Server to reach the AmpCon/DHCP/TFTP server to complete the system installation.

Figure 2. LACP Fallback Applied in MLAG Networks

With LACP fallback feature, Spine switches allow the system to bring up the LAG (before receiving any LACP PDUs from the Leaf switch/ToR/Server) and keeps one of the member ports active until receiving an LACP PDU on any one of the member ports from the Leaf switch/ToR/Server. This allows the newly installed Leaf switches/ToR/Server to establish a connection, reaching the AmpCon server to get provisioned.

Timer

The switch starts LACP fallback timer when LACP configurations are finished and LACP fallback is enabled on the switch. If no LACP PDUs are received from the peer device on any of the member ports of the LAG when this timer expires, the switch will set the LAG to fallback mode and bring up one member port to active (up) state.

The switch will not exit LACP fallback mode until it receives an LACP PDU from the peer device.

Use the following command to configure the LACP fallback timer. The default value is 10 seconds.

set interface aggregate-ethernet <lag-interface> aggregated-ether-options lacp fallback timeout <timer>

Port Election

Since only one member port of the LAG can be chosen as the active port, the port election rules are:

• The port with lowest LACP port priority value will be chosen as the active port.
• If LACP port priorities are the same, port with lowest port index will be chosen as the active port.

The following command can be used to set priority of the LACP member port:

set protocols lacp interface <physical-port> priority <priority-value>

For non-MLAG environment, the port election rule is as described above.

For MLAG environment, port election is first performed on each MLAG peer device. Then the elected ports information is exchanged between MLAG peers. And finally, an active port is elected from these two ports according to the port election rules described above.

LACP Fallback supports fallback port preemption. A port with a higher priority can always preempt the active fallback port as the new active fallback port. During LACP fallback mode, when a non-fallback port is configured with a higher priority, the system will perform port re-election, as a result, the higher priority port will replace the elected port as the active port.

Configuring LACP Fallback

Prerequisite Configurations

1. Create a LAG interface.
2. Add member ports to the LAG interface.
3. Enable LACP protocol on the LAG interface.

Enable LACP Fallback

• Enable LACP fallback.

set interface aggregate-ethernet <lag-interface> aggregated-ether-options lacp fallback enable <true | false>

• (Optional) Configure LACP fallback timer. The default value is 10 seconds.

set interface aggregate-ethernet <lag-interface> aggregated-ether-options lacp fallback timeout <timer>

Configuration Example

As shown in the following figure, Pica8 Switch and the PXE Booting Clients (ToR or Server) are connected through an LACP LAG interface. You can enable LACP fallback function on Pica8 Switch, which will allow the PXE Booting Client to connect to the DHCP/TFTP Server through one active port on Pica8 Switch during the PXE phase.

Procedure

Step 1       Create a LAG interface ae1, add physical ports and enable LACP protocol on the LAG.

admin@XorPlus# set interface aggregate-ethernet ae1
admin@XorPlus# set interface gigabit-ethernet ge-1/1/1 ether-options 802.3ad ae1
admin@XorPlus# set interface gigabit-ethernet ge-1/1/2 ether-options 802.3ad ae1
admin@XorPlus# set interface gigabit-ethernet ge-1/1/3 ether-options 802.3ad ae1
admin@XorPlus# set interface aggregate-ethernet ae1 aggregated-ether-options lacp enable true

Step 2       Enable LACP fallback on LAG interface ae1.

admin@Xorplus# set interface aggregate-ethernet ae1 aggregated-ether-options lacp fallback enable true

Step 3       Commit the configurations.

admin@XorPlus# commit

Step 4       Verify the configurations.

Run the command run show interface aggregate-ethernet <lag-interface> to display configuration information and status of the LAG interface, “Fallback: Enabled” shows that LACP fallback is enabled.

admin@XorPlus# run show interface aggregate-ethernet ae1
Physical interface: ae1, Enabled, error-discard False, Physical link is Up
Interface index: 80, Mac Learning Disabled
Port mode: access
Description:
Link-level type: Ethernet, MTU: 1518, Speed: 1Gb/s, Duplex: Auto
Source filtering: Disabled, Flow control: Disabled
Auto-negotiation: Disabled
Interface flags: SNMP-Traps Internal: 0x0
Current address: e0:07:1b:c9:20:9b, Hardware address: e0:07:1b:c9:20:9b
Traffic statistics:
  5 sec input rate 952 bits/sec, 1 packets/sec
  5 sec output rate 624 bits/sec, 1 packets/sec
  Input Packets............................25877
  Output Packets...........................29275
  Input Octets.............................2445697
  Output Octets............................2509890
Aggregated link protocol: LACP
Fallback: Enabled
Minimum number of selected ports: 0
  Members        Status          Port Speed
  ---------      ----------      ----------
  ge-1/1/1      Up(active)      1Gb/s  
  ge-1/1/2      Up(active)      1Gb/s 
  ge-1/1/3      Up(active)      1Gb/s