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MLAG - Switch does not reply to the first Gratuitous ARP request received on ISC port.

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TitleMLAG - Switch does not reply to the first Gratuitous ARP request received on ISC port.
Symptoms
  • Summit switches configured with MLAG does not reply to the first ARP request received on an ISC port
Environment
  • EXOS All
  • MLAG
Cause
  • When ARP request is received on ISC port, and the switch does not have FDB entry of the sending host, the switch does not reply to the ARP request.
  • Because of this behavior, the first ARP request from the host is not replied, and the host has to retransmit the ARP request.

Detailed explanation with the sample topology below:

User-added image


 
  • By default, FDB learning is disabled on the ISC port as per the MLAG implementation.
  • With the topology above, When ARP is received in SW1 from SW3, the ARP will be flooded to ISC port in hardware and a copy will be lifted to SW1 CPU for learning.
  • Note: ARP will be flooded in hardware faster than the FDB getting synced by software between the MLAG peers.
  • SW2 will not update the ARP table because the corresponding FDB is not yet present in the switch. Hence the ARP entry is not getting updated on the first packet.
  • Later FDB will be check pointed by SW1 and FDB will be programmed in SW2 as well.
  • Now when ARP entry (second packet) is received on ISC port, SW2 will update the entry in ARP table.
Resolution
  • The feature request CR# xos0067476 is created to checkpoint IPARP entries between MLAG peers similar to FDB and if this CR is fixed, the issue will be taken care.
  • The CR xos0067476 will be implemented in upcoming releases.
           
 
Additional notes

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