OS10 Enterprise Edition User Guide Release 10.4.1.0

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LACP fallback

LACP fallback allows an active LACP interface to establish a port-channel before receiving LACP PDUs from the peer.

This feature is useful in environments where Preboot Execution Environment (PXE) servers are connected to switch with LACP port-channel.

Whenever a PXE server reboots, both the port-channel and ports go down. While rebooting, the ports come up, but not the port-channel. LACP fallback enables the port-channel to be up and keeps sending packets to the PXE server.

The LACP fallback feature adds a member port to LACP port-channel if it does not receive LACP PDUs from the peer for a particular period of time. You can set the timer using the lacp fallback timeout timer-value command.

The member port becomes active and sends packets to the PXE server.

When the switch starts receiving LACP PDU, OS10 ungroups the statically added member port from LACP port-channel and resumes with normal LACP functionality..

When you enable LACP fallback, the port that comes up is selected based on the following:
  • LACP port priority configuration allows deterministic port allocation. The port with the least priority is placed in the active state when a port-channel is in LACP fallback mode.
  • If all the ports in a port-channel have same port priority, the switch internally compares the interface names by base name, module number, port number, and then selects the lowest one to be active. For example, Ethernet 1 is less than Ethernet 2 and hence Ethernet 1 becomes active.
  • In a VLT network, if the interface name is the same on both the VLT peers, then the port in switch with lower system MAC address becomes active.
Limitations
  • OS10 switches cannot be a PXE client irrespective of whether it acts as a VLT peer or ToR switch.
  • If you are configuring LACP fallback in a VLT domain, configure lacp fallback commands in both the VLT peers.
  • If you do not enable LACP fallback in one of the VLT peers, or configure different time-out values in the peers, then the switch might behave differently.
  • The LACP fallback feature adds or groups a member port to the port channel only when the switch does not receive LACP PDUs from the peer, to make the link connected to the PXE client device as operational. As PXE clients handle untagged DHCP request, you need to configure the LACP fallback only on an untagged VLAN to reach the DHCP/PXE server.
  • After the LACP fallback election, if a port with lower priority port is configured to be part of the same port-channel, it would trigger re-election.
Configure LACP fallback
  1. Enable LACP fallback with the lacp fallback enable in port-channel INTERFACE mode.
  2. Set a timer for receiving LACP PDUs using lacp fallback timeout timer-value in port-channel INTERFACE mode.
Example configuration
OS10# configure terminal
                                 OS10(config)# interface port-channel 1
                                 OS10(conf-if-po-1)# lacp fallback enable
                                 OS10(conf-if-po-1)# lacp fallback timeout 20
                              
View LACP fallback configuration
OS10# show port-channel summary
                                 
                                 Flags:  D - Down    I - member up but inactive    P - member up and active
                                 
                                 U - Up (port-channel) F - Fallback enabled
                                 
                                 --------------------------------------------------------------------------------
                                 Group Port-Channel      Type     Protocol  Member Ports
                                 --------------------------------------------------------------------------------
                                 
                                 1    port-channel1  (UF) Eth      DYNAMIC   1/1/10(P) 1/1/11(I)
                                 
                                 
                                 
                              

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