• Configuring LACP Fallback Individual Ports on Arista Switches

 
 
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LACP Fallback Individual Ports

Feature Overview

  • LACP Fallback Individual Ports is a feature introduced in EOS 4.13.0 that allows all ports in a port- channel to fallback to individual switch ports when negotiation fails
  • The feature is applied to the port-channel interface and consists of two configuration elements which will be described in the following sections
    • Setting the port-channel to individual fallback
    • Setting the fallback timeout

 

Feature Operation

LACP Fallback Individual Ports use cases

  • This feature is useful for servers with multiple NICs where it is difficult to predict which NIC the server might use for PXE boot.

Summary of individual fallback mode

  • When individual fallback is configured, all ports in a port-channel will be brought up as individual switch ports when LACP negotiation fails and the fallback timeout expires
  • While in individual mode, ports will continue to send and receive LACPDU messages in an effort to re-establish LACP negotiation with the partner switch or server
  • If a LACPDU is received on any port in the port-channel, all ports of the port-channel will cease to be in individual mode
  • The fallback timer is reset if the port-channel interface is shutdown then re-enabled shut/ no shut
  • The fallback timer is not affected if the physical ports flap, or if ports are removed from the port-channe
  • If the fallback timer is changed, the new expiration time will be set 
    • If timer is in the past it will fire immediately
  • Configuring a fallback timeout on a static port-channel will have no effect
  • Under both individual fallback and static fallback, it is unsupported to configure both min-links and fallback
  • All ports configured in a port-channel use the LAG’s ACL instead of their own ACL
    • This applies even when a port is in individual mode
  • When in individual mode, the physical interface configuration will be used instead of the port-channel configuration
    • This behavior allows for servers undergoing PXE boot to join a specific access VLAN used for server builds
    • Later, when LACP is established perhaps after server build is complete, the port-channel configuration will be utilized which may be configured as an access port on a different VLAN or as a trunk port
  • Operation in MLAG
    • The fallback mode of MLAG peers must match fallback to be enabled
    • If the fallback mode is inconsistent between peers for two minutes, a warning message is logged
  • CLI changes
    • The Following command is available in interface port-channel configuration mode
[no|default] port-channel lacp fallback [static|individual]
  • Using the individual keyword configures individual fallback mode
    • Leaving out the fallback type or using the static keyword activates static fallback mode
    • Issuing “no” or “default” keywords will disable all fallback modes
  • The show port-channel summary output will indicate status of fallback, note capitalization 
a – LACP active, f – fallback configured, ^ - individual fallback
 	Po10(U)            LACP(af^)     Et1(PG+) Et2(PG+)
a – LACP active, F – fallback enabled, ^ - individual fallback 
	Po10(D) LACP(aF^) Et1(G-) Et2(G-)
a – LACP active, f – fallback configured, * - static fallback
	Po10(U) LACP(af*) Et1(PG+) Et2(PG+)
a – LACP active, F – fallback enabled, * - static fallback 
	Po10(D) LACP(aF*) Et1(G-) Et2(G-)

 

Example

In the following example, the port-channel and physical interface are configured to access different VLANs

  • If LACP negotiation is successful, Ethernet 7-8 would be added as a member of Port-channel10, and will bridge frames on VLAN 10, frames tagged with other VLANs are dropped
  • If LACP negotiation is unsuccessful and the fallback timer expires, Ethernet7-8 will enter individual mode. It will then bridge frames on VLAN 15, and drop frames tagged with other VLANs
interface Port-Channel10
        switchport access vlan 10
        port-channel lacp fallback individual
        port-channel lacp fallback timeout 95
interface Ethernet7
        switchport access vlan 15
        channel-group 10 mode active
interface Ethernet8
        switchport access vlan 15
        channel-group 10 mode active

Example of normal port-channel with LACP established

7050-01(config)#sh port-channel
Port Channel Port-Channel10:
  Active Ports: Ethernet7 Ethernet8

7050-01(config)#sh port-channel summary

                 Flags
------------------------ ---------------------------- -------------------------
  a - LACP Active          p - LACP Passive           * - static fallback
  F - Fallback enabled     f - Fallback configured    ^ - individual fallback
  U - In Use               D - Down
  + - In-Sync              - - Out-of-Sync            i - incompatible with agg
  P - bundled in Po        s - suspended              G - Aggregable
  I - Individual           S - ShortTimeout           w - wait for agg

Number of channels in use: 1
Number of aggregators:1

   Port-Channel       Protocol     Ports
------------------ --------------- --------------------------------------------
   Po10(U)             LACP(af^)       Et7(PG+) Et8(PG+)
7050-01(config)#sh int port-Channel 10
Port-Channel10 is up, line protocol is up (connected)
  Hardware is Port-Channel, address is 001c.731c.46ef
  Ethernet MTU 9214 bytes , BW 20000000 kbit
  Full-duplex, 20Gb/s
  Active members in this channel: 2
  ... Ethernet7 , Full-duplex, 10Gb/s
  ... Ethernet8 , Full-duplex, 10Gb/s
  Fallback mode is: individual
  Fallback mode status: Monitoring
  Fallback timeout is 95 seconds
  Up 3 seconds
  10 link status changes since last clear
  Last clearing of "show interface" counters never
  5 minutes input rate 125 bps (0.0% with framing overhead), 0 packets/sec
  5 minutes output rate 355 bps (0.0% with framing overhead), 0 packets/sec
     127 packets input, 21429 bytes
     Received 0 broadcasts, 127 multicast
     0 input errors, 0 input discards
     510 packets output, 66163 bytes
     Sent 0 broadcasts, 510 multicast
     0 output errors, 1 output discards

Example of port-channel in individual fallback mode

7050-01(config)#sh port-channel
Port Channel Port-Channel10:
  No Active Ports
  Configured, but inactive ports:
       Port         Reason unconfigured
    --------------- --------------------------------
       Ethernet7    in lacp fallback individual mode
       Ethernet8    in lacp fallback individual mode
7050-01(config)#sh port-channel summary

                 Flags
------------------------ ---------------------------- -------------------------
  a - LACP Active          p - LACP Passive           * - static fallback
  F - Fallback enabled     f - Fallback configured    ^ - individual fallback
  U - In Use               D - Down
  + - In-Sync              - - Out-of-Sync            i - incompatible with agg
  P - bundled in Po        s - suspended              G - Aggregable
  I - Individual           S - ShortTimeout           w - wait for agg

Number of channels in use: 1
Number of aggregators:1

   Port-Channel       Protocol     Ports
------------------ --------------- --------------------------------------------
   Po10(D)             LACP(aF^)       Et7(G-) Et8(G-)
7050-01(config)#sh int port-Channel 10
Port-Channel10 is down, line protocol is lowerlayerdown (notconnect)
  Hardware is Port-Channel, address is 001c.731c.46ef
  Ethernet MTU 9214 bytes
  Full-duplex, Unconfigured
  Active members in this channel: 0
  Fallback mode is: individual
  Fallback mode status: Enabled
  Fallback timeout is 95 seconds
  Down 1 minutes, 47 seconds
  7 link status changes since last clear
  Last clearing of "show interface" counters never
  5 minutes input rate 107 bps (- with framing overhead), 0 packets/sec
  5 minutes output rate 339 bps (- with framing overhead), 0 packets/sec
     80 packets input, 13590 bytes
     Received 0 broadcasts, 80 multicast
     0 input errors, 0 input discards
     327 packets output, 42365 bytes
     Sent 0 broadcasts, 327 multicast
     0 output errors, 0 output discards

 

Understanding fallback timeout

  • If no LACPDUs are received for the number of seconds configured as the fallback timeout, EOS will check if LACP negotiation has expired
  • If LACP negotiation has expired the ports will fall back to individual mode
  • If LACP negotiation has not expired another fallback timer is scheduled at a value equal to the fallback timeout
  • Example: fallback timeout is set to 50 seconds and LACP negotiation is set to default 90 seconds
    • After 50 seconds, fallback timer is rescheduled because LACP negotiation has not expired
    • After an additional 40 seconds (90 total) LACP negotiation expires and become inactive
    • Another 10 seconds passes (100 seconds total) when the fallback timer expires and the ports fallback to individual
    • It becomes important to understand this behavior when using fallback timeout values that are less than the LACP negotiation timeout
    • When setting the fallback timeout to 90 seconds and LACP negotiation timeout is also set to default 90 seconds, the fallback timeout does not appear in the running configuration under the port-channel interface
    • For these reasons it does not make sense to configure a fallback timer that is less than the LACP negotiation timeout value
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