Dell PowerSwitch S4128F-ON EMC SmartFabric OS10 User Guide Release 10.5.0 - Page 412

LACP fallback LACP fallback allows downstream devices, like servers which are connected to ports of a switch configured as LACP, to establish a link when the system is not able to finalize the LACP handshake. For example, when servers boot in PXE mode, the server cannot exchange LACP PDUs and the switch does not enable the ports. 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. When you enable LACP fallback, the switch starts a timer. If the timer expires before LACP completes, then the switch selects one port of the port group and makes it operational. You can set the timer using the lacp fallback timeout timer-value command. 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. The server uses the fallback port to finalize the PXE-boot process. When the server starts with the operating system, the process completes the LACP handshake and the fallback port reunites the other members. 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 1/1/1 is less than ethernet 1/1/2 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. 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. 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. • 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 that is 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 reelection. Configure LACP fallback 1. Enable LACP fallback with the lacp fallback enable command in port channel INTERFACE mode. 2. Set a timer for receiving LACP PDUs using lacp fallback timeout timer-value in port channel INTERFACE mode. 3. (Optional) Enable or disable LACP fallback port preemption using lacp fallback preemption {enable | disable} 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 OS10(conf-if-po-1)# lacp fallback preemption enable 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 412 Layer 2