| Note: | This feature is supported only in the IPv4 operating mode. |
The address mask protocol enables you to configure your network so that the hosts in a particular network can dynamically discover the mask, thus avoiding the need for each host to have a statically configured subnet mask.
To enable a host to use the address mask protocol, set one or more of the following address mask attributes.
|
Attribute |
Description |
|---|---|
|
IPMASKNETMASK |
The 32‑bit address mask for this network, used to find the subnet number. For additional information about address masks, see Subnetting. |
|
IPMASKNETADDR |
The network address for which the address mask information applies. For additional information about address masks, see Subnetting. |
|
IPMASKCONFIG |
Identifies how the host will behave in exchanging subnet mask addressing information among other hosts in the network. |
|
IPMASKRETRYLIMIT |
Identifies how many times an enabled host should retransmit a request when a reply is not received from the agent in the subnet. |
The address mask attributes that you set will be stored in an Address Mask table, indexed by IPMASKNETADDR.
To enable the address mask protocol, you must set the IPMASKCONFIG attribute to configure a <state> for each host on the network. The <state> you assign enables you to control the behavior of the host in regard to the exchange of subnet mask addressing information among other hosts in the network.
| Note: | The TCPIP IDENTITY command automatically defaults the <state> of a host to static, until an SNMP SET command is issued to define a different value for the <state>. |
Enabling a Host to Exchange Subnet Mask Address Information
To enable a host to exchange subnet mask address information, issue the following command from your system console:
NW SNMP SET IPMASKCONFIG <ip address> = <state>
The variables are described as follows.
|
Variable |
Description |
|---|---|
|
<IP address> |
The local IP address of the host that you want to participate in dynamic address mask discovery. |
|
<state> |
Any one of the following values:
|
If this command is successful, you receive the following response:
SNMP OBJECTS REQUEST ID = 0 ERROR STATUS = 0 ERROR INDEX = 0 OBJECT ID = IPMASKCONFIG 1.3.6.1.4.1.223.5.1.1.5.1.3.<IP address> OBJECT DESC = <1,2,3,or 4>
| Note: | Verify that the ERROR STATUS and ERROR INDEX fields shown in the system response are set to 0 (zero). If any other value is shown, it is an indication that the SNMP command has failed; for additional information, refer to the SNMP SET command responses listed in the Networking Commands and Inquiries Help. |
The OBJECT DESC field indicates the setting of the IPMASKCONFIG attribute. The following table identifies the available configurations.
|
OBJECT DESC Values |
When set to this value, the host will act as . . . |
|---|---|
|
1 |
Enabled |
|
2 |
Static_Agent |
|
3 |
Agent |
|
4 |
Static |
The <state> that you assign to the IPMASKCONFIG attribute identifies how the host will behave in exchanging subnet mask addressing information among other hosts in the network.
IPMASKCONFIG Attribute Values identifies the <states> that can be assigned to a subnet mask.
Table 5. IPMASKCONFIG Attribute Values
|
If you set the state to . . . |
And you issue a CLEAR CALL or ESTABLISH CALL command (with the AUTOINIT attribute set to TRUE), then the host will . . . |
If while operating, you modify the state of that network, the host will . . . |
|---|---|---|
|
Enabled |
Send address mask requests and receive address mask replies from the agent in the network. When address mask requests are being sent to another host in the network, the initiating host will send requests based on the retry limit set for that host. Requests will continue to be sent until one of the following occurs:
|
Receive address mask replies from the agent in the network, providing you have reinitialized the agent in your network. |
|
Agent |
Send unsolicited replies to other hosts in the network, and answer requests from other hosts in the network. However, the host will ignore all incoming replies received from other hosts in the network. |
Respond to all incoming requests received from other hosts in the network. |
|
Static_Agent |
Have the same functionality as if the state was set to Agent except the host will only answer specific requests from other hosts; it will not send out unsolicited replies to other hosts in the network. |
Respond to all incoming requests received from other hosts in the network. |
|
Static |
Ignore all requests/replies from other hosts in the network. In addition, the host will not send out any requests for information about other hosts in the network. A static host uses the address mask that was statically configured through the TCP/IP IDENTITY command or the SNMP SET command using the IPMASKNETMASK attribute. |
Ignore all requests/replies from other hosts in the network. In addition, the host will not send out any requests for information about other hosts in the network. A static host uses the address mask that was statically configured through the TCP/IP IDENTITY command or the SNMP SET command using the IPMASKNETMASK attribute. |
Tips on Identifying a Value for the IPMASKCONFIG Attribute
There are some things you should keep in mind when setting a value for the IPMASKCONFIG attribute. When enabling a host to act as
-
Enabled
A host must be set to Enabled to discover its subnet mask from the agent in the network. If you are setting the <state> of a host in the subnet to Enabled, you have an additional option to determine how many times the enabled host should retransmit a request when a reply is not received from the agent in the network.
To set the number of address mask requests issued by an enabled host, set the IP Mask Retry Limit attribute through the SNMP SET command. To do so, issue the following command from the system console:
NW SNMP SET IPMASKRETRYLIMIT <IP address> = <retry limit value>
where <retry limit value> is a range from 1 to 255.
For example, if you issue the following command from the system console, you are enabling the host to retransmit a request for address mask information ten times before the request is dropped.
If you do not identify a value and no reply is received for this attribute, a default value of 5 will be used. If the number of retries expires, it is an indication that there might be a configuration problem in the subnet (for example, an agent is not defined).
NW SNMP SET IPMASKRETRYLIMIT <IP address> = 10
To inquire on the current setting of the IPMASKRETRYLIMIT attribute for any enabled host on a network, issue the following command from the system console:
NW SNMP GET IPMASKRETRYLIMIT <IP address>
-
Agent
You must statically configure an address mask for the host that is to act as an agent by setting the IPMASKNETMASK attribute through the SNMP SET command.
For specific details on how to statically configure an address mask for a host in your network, see Subnetting.
Set the IPMASKCONFIG attribute to Agent (through issuing the SNMP SET command).
Note: It is recommended that the host that is acting as an agent for the subnet is either the first or last host to be initialized in the network. You can configure more than one host in the network to act as an agent to the rest of the hosts in the network; however, be advised that an enabled host will only accept the "first" reply received from an agent as its address mask.
Note: If you change the value of the IPMASKCONFIG attribute for a host from Agent to another state (Enabled, Static, or Static_Agent), you must identify another host in the network to act as the agent. For additional information on changing the value of the IPMASKCONFIG attribute, see "Changing the Setting of the IPMASKCONFIG Attribute" later in this section. -
Static
If you do not want a host in the network to participate in the exchange of subnet mask addressing information among other hosts in the network, set the IPMASKCONFIG attribute for that host to Static. This enables the host to ignore all requests and replies from other hosts in the network. In addition, when set to static, the host will not request information from the agent in the network.
Static is the default configuration setting for the IPMASKCONFIG attribute. If you want the host to act as static, you do not need to issue the SNMP SET command. The TCPIP IDENTITY command will automatically default the IPMASKCONFIG attribute to Static until an SNMP SET command is issued to define a different value for the IPMASKCONFIG attribute.
If you change the <state> of a host's configuration from Enabled, Static_Agent, or Agent to Static, the host will maintain the same subnet mask from the previous <state>.
-
Static_Agent
If you set a host to Static_Agent, it will have the same functionality as that of a host set to Agent, except the host will only answer specific requests from other hosts in the network; it will not send out unsolicited replies to other hosts in the network upon initialization.
You can change the <state> of the IPMASKCONFIG attribute for any host in the network as often as you want; however, in doing so, you could potentially introduce network problems. Therefore, it is important that you understand the implications of modifying the <state> before you alter your initial configuration. For details, see "Changing the Setting of the IPMASKCONFIG Attribute" later in this section.
Changing the Setting of the IPMASKCONFIG Attribute
Changing the setting of the IPMASKCONFIG attribute is the same as initially setting this attribute for the host through the SNMP SET command. For example, to change the setting of the IPMASKCONFIG attribute from a value of Static to Enabled, issue the following command from your system console:
NW SNMP SET IPMASKCONFIG <IP address> = ENABLED
where <IP address> is the IP address of the host for which you want to change the setting of the IPMASKCONFIG attribute.
A change will take effect immediately; however, the behavior of the host for a particular <state> can be different depending on how the <state> is applied.
For example, if you change a host from Static to Enabled and do not issue a CLEAR CALL command (with the Auto Init attribute set to TRUE) or an ESTABLISH CALL command on the appropriate LAN connection, the host will accept replies from the agent in the network (provided the agent has been reinitialized), but will not send requests to the agent. The only way to get the newly enabled host to send a request to the agent is to issue one of the following commands from the system console:
-
NW CLEAR CALL command (with the Auto Init attribute set to TRUE) on the appropriate LAN connection
-
NW ESTABLISH CALL command on the appropriate LAN connection
For a complete description of each command, refer to the Networking Commands and Inquiries Help.
Selecting a New Agent for the Network
If you are selecting a new agent for your network, you must change the <state> of two hosts in the network. To select a new agent for the network, perform the following steps:
-
Change the value of the IPMASKCONFIG attribute of the host that you want to act as an agent in the network from its current setting to Agent, using the SNMP SET command.
-
Verify that the host that you want to act as the new agent has a statically configured address mask. If it does not, you can statically configure an address mask by issuing the SNMP SET command, using the IPMASKNETMASK attribute. For additional details, see Subnetting.
-
Change the setting of the old Agent's IPMASKCONFIG attribute from Agent to another value (for example, Enabled), using the SNMP SET command.
If you issue a CLEAR CALL command on the appropriate LAN connection of the host that is acting as the new agent, it will send out unsolicited replies with the new mask.
Keep in mind however, that the first reply received from an agent is the one and only subnet mask that is accepted by the enabled host (as per RFC 1122). To get an enabled host to accept a new subnet mask (for example, if the new agent in the network has a different subnet mask than the previous agent), you must do one of the following to each host in the network:
-
Reissue the SNMP SET command to set the IPMASKCONFIG attribute to Enabled on each host that you want to accept the new subnet mask, and issue a CLEAR CALL command or an ESTABLISH CALL command on the agent.
-
Issue a CLEAR CALL command (with the Auto Init attribute set to TRUE) or an ESTABLISH CALL command on the appropriate LAN connection of the host that you want to accept the new subnet mask.
-
Statically configure the new mask for the host by issuing the SNMP SET IPMASKNETMASK command.
Address Mask Log Messages
The following address mask log messages might be displayed in your system log:
-
If you set a new address mask using an OI command, the log displays the following message:
MASK <subnet mask> PROVIDED IN SNMP SET COMMAND FOR IP ADDRESS HAS REPLACED MASK <subnet mask> FOR IP ADDRESS <IP address> IN IP_ADDR_TABLE
-
If a mask is set by an address mask reply, the log displays the following message:
MASK <subnet mask> PROVIDED IN ADDRESS MASK REPLY FOR NETWORK ADDRESS <IP address> HAS REPLACED MASK <subnet mask> IN IP_ADDR_TABLE
-
If a request is received by a host, and the IPMASKCONFIG attribute is set to Agent - but the mask was not set through the SNMP SET command, no reply is sent and the log report displays the following error message:
ICMP ADDRESS MASK REQUEST RECEIVED ON NETWORK <IP address>; MASK IN MASK TABLE NOT SET BY OI, CAN'T USE THIS MASK TO REPLY
