what is the use of -a option of ISC relay agent

[ashok singh]

Hi All,
  
  The ISC relay agent when started with -a option will append the  interface name through which the DHCP request came and forward it to  the DHCP Server. At the DHCP Server based on this interface name we can  do address allocations.
  
  This address allocation is anyways done implicitly based on giaddr field of incoming packet by DHCP Server.
  
  So my question is how this -a option can be useful?
  
  Regards,
  Ashok
  
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