• Configuring Port Channel LACP Fallback on Arista Switches

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The Port-Channel Fallback mode in Arista switches allows an active LACP interface to establish a Port Channel/LAG before it receives LACP PDU’s from its peer.

This feature is useful in environments where customers have Preboot Execution Environment (PXE) Servers connected with a LACP Port Channel to the Ethernet switch.  Since PXE images are very small, many operating systems are unable to leverage LACP during the preboot process.  The Server NICs do not have the capability to run LACP without the assistance of a fully functional OS, and during the PXE process they are independent and have no knowledge of the other NIC or a method to form an LACP connection.  Both the NIC’s on the server will be active and are sourcing frames from their respective MAC addresses during the initial boot process.  Simply keeping both the ports in the LAG active will not solve the problem because packets sourced from the MAC address of NIC-1 can be returned to the port on which NIC-2 is attached, which will cause NIC-2 to drop the packets (due to MAC mismatch).

With the LACP fallback feature, the switch allows the server to bring up the LAG without receiving LACP PDU’s, and fallback to a single port active after the expiry of the timeout period.  This allows the PXE boot server to establish a connection over one Ethernet port, download its boot image and continue the booting process.  When the network boot process is complete, the Server fully forms an LACP Port Channel/LAG.

Below are the steps to enable LACP Fallback in Arista switches.

1. Configure “lacp port-priority <priority value>” under Interface Ethernet configuration (Optional)

  • This command sets the aggregate port priority for the configuration mode interface.
  • LACP port priority determines the port that is active in a LAG in fallback mode.
  • Numerically lower values have higher priority. Priority numbers range from 0 to 65535.
  • The default is 32768. Interfaces with higher priority numbers are placed in standby mode before interfaces with lower priority numbers.
  • This command is optional, but it gives a deterministic way to allocate which port in the port-channel will be active after expiry of the timer

2. Configure “Port-channel lacp fallback”, under Interface – Port Channel.

  • This command enables LACP fallback feature.
  • The “no port-channel lacp fallback” disables the feature

3. Configure “port-channel lacp fallback timeout <period>”, under Port Channel configuration.

  • This command specifies the period a LAG in fallback mode remains active without receiving an LACP PDU from its peer.
  • Upon timer expiry, the interface reverts to static mode with one active port.
  • The default fallback timeout period is 90 seconds.  The period value range is between 0 – 100 seconds

LACP Fallback with MLAG

With MLAG LACP Fallback works exactly the same way as described above. Both switches in the MLAG should be configured identically on both peers (for interfaces facing downstream to the server).  The MLAG peers will coordinate to bring up only the lowest-priority port.

LACP Port Priority

The LACP Port Priority configuration is optional, but it allows for deterministic port allocation when it comes to which port will be placed into the port-channel.  The port that has the least priority configured will be placed into active state when the port-channel is in Fallback mode.  In the case where all ports in a port-channel have the same port-priority (default), the switch has an internal algorithm that compares interface names (Ethernet 1, Ethernet 2 etc.) and returns the equivalent of less than, equal and greater than.  The algorithm compares two interface names by base name, then by module number (if any) and finally by port number (if any) and selects the port that’s the lowest; i.e. Ethernet1 < Ethernet2, then we choose Ethernet1 to bring up.

In the case of an MLAG deployment, if the interface name is the same on both switches, then the switch with a lower System Mac Address will bring its local interface up.


Consider the following diagram and the subsequent switch configuration:

port channel

Interface Port-Channel20
 Port-channel lacp fallback
 Port-channel lacp fallback timeout 5

Interface Ethernet1
 Channel-group 20 mode active
 Lacp port-priority 15000

Interface Ethernet 2
 Channel-group 20 mode active
 Lacp port-priority 16000

LACP Fallback Steps

1.  The PXE Client server boots up.  The Arista switch detects link up and brings up both the ports in the LAG listening for LACP PDU’s.  During this time the switch will only process LACP and LACP PDU’s, and all other packets will be dropped.

switch(config-if-Et1)#show port-channel 20 detail
Port Channel Port-Channel20:
 No Active Ports
 Configured, but inactive ports:
 Port                Time became inactive    Reason unconfigured 
 ------------------- -------------------------- ------------------- 
 Ethernet1           14:53:10                LACP Wait 
 Ethernet2           14:53:13                LACP Wait

2.  The switch will wait for 5 seconds for any LACP PDU’s (as the fallback timeout is configured for 5 seconds). If none are received the LAG/Port-Channel will enter into Fallback mode.  During the Fallback mode only Ethernet Port 1 is active (since the LACP Port-Priority for Ethernet 1 is lower then Ethernet 2).  Both Ethernet 1 and Ethernet 2 will continue to listen for LACP PDU.  Any packets other then LACP PDU’s received on Ethernet 2 will be dropped during this time.

switch(config-if-Et1)#show port-channel 20 detail
Port Channel Port-Channel20:
 Active Ports:
 Port           Time became active       Protocol       Mode      Fallback 
 -------------- ----------------------- -------------- ------------ -------- 
 Ethernet1      14:56:00                 LACP           Active    Enabled 

 Configured, but inactive ports:
 Port                Time became inactive 
 ------------------- -------------------------- 
 Ethernet2          14:56:00 

 Port             Reason unconfigured 
 ------------------- ----------------------------------------------- 
 Ethernet2    lacp fallback intf limit exceeded

3.  The PXE Client server now has one network interface up and active, and will send packets for DHCP Server information.

4.  After receiving information from the DHCP Server, the Server will proceed to boot from the TFTP/NFS server.

5.  After the server has fully loaded its boot image it will start sending LACP PDUs.  The Arista switch receives the LACP PDU from the Server and as a result activates the Port-Channel/LAG.

switch(config-if-Et1)#show port-channel 20 detail
Port Channel Port-Channel20:
 Active Ports:
 Port                Time became active       Protocol    Mode 
 ------------------- ------------------------ -------------- ------ 
 Ethernet1           14:53:02                 LACP        Active
 Ethernet2          14:52:57                 LACP        Active

See also Configuring Port Channel LACP Fallback Individual Ports on Arista Switches


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