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Explain the “M” and “O” bit in IPv6 DHCP server configuration. What are the associated flags ? 

Jul 01, 2014 07:29 AM

This article applies to Aruba Mobility Controllers running ArubaOS version



A DHCPv6 client will know when it wants to use DHCPv6 either based upon instruction of an attached router, or when no default gateway is present. The default gateway has two configurable bits in an Router Advertisement (RA) available for this purpose:

• ‘M’ bit - "Managed address configuration" flag. When set, it indicates that the client may use DHCPv6 to retrieve a Managed IPv6 address from a DHCPv6 server. If the M flag is set, the O flag is
redundant and can be ignored because DHCPv6 will return all available configuration information.
• ‘O’ bit -  "Other configuration" flag. When set , indicates that other configuration information is available via DHCPv6. Examples of such information are DNS-related information or information on other servers within the network.

When a router sends an RA with ‘O’ bit set, but does not set the ‘M’ bit, the client can do Stateless Address Autoconfiguration (SLAAC) to obtain its IPv6 address, and use DHCPv6 for obtaining additional information. (An example of additional information is DNS). This mechanism is well known as Stateless DHCPv6, because the DHCPv6 server does not need to keep track of the client address bindings.
Note: If neither M nor O flags are set, this indicates that no information is available via DHCPv6.

Cases :

M = 0, O = 0  : Client uses RA to get non-link-local address and other settings. Even DHCPv6 server exists, client ignores it.
M = 1, O = 0  : Client uses DHCPv6 for both address and other configuration settings. This combination is known as DHCPv6 stateful, in which
M = 1, O = 1                   
                     DHCPv6 is assigning stateful address to client. If RA includes 
                     network prefix, client also gets stateless address.
M = 0, O = 1  : Client gets stateless address from RA. DHCPv6 only assign other configuration settings to client. This combination is known as DHCPv6 stateless.
A Packet capture showing the case M = 0, O = 1
  1. In the Router Advertisement the fags M = 0, O = 1 is set as below
rtaImage 1.png
  2.   Since M = 0 , client do Stateless Address Autoconfiguration (SLAAC) to obtain its IPv6 address and Since O = 1 , it request for information like the Name sever to the DHCPv6 Server.
rtaImage 2.png
3.   The DHCP server in response sends back the Name server information back to the client.
rtaImage 3.png


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