Additional Configurations for DHCP Clients
Specifying the Maximum Number of DHCP Clients Per Interface
By default, there is no limit to the number of DHCP local server or DHCP relay clients allowed on an interface. However, you can override the default setting and specify the maximum number of clients allowed per interface, in the range 1 through 500,000. When the number of clients on the interface reaches the specified limit, no additional DHCP Discover PDUs or DHCPv6 Solicit PDUs are accepted. When the number of clients subsequently drops below the limit, new clients are again accepted.
The maximum number of DHCP (and DHCPv6) local server clients
or DHCP (and DHCPv6) relay clients can also be specified by Juniper
Networks VSA 26-143 during client login. The VSA-specified value always
takes precedence if the interface-client-limit statement
specifies a different number.
If the VSA-specified value differs with each client login, DHCP uses the largest limit set by the VSA until there are no clients on the interface.
To configure the maximum number of DHCP clients allowed per interface:
For DHCP local server and DHCP relay agent, you can use
either the interface-client-limit statement or the client-discover-match incoming-interface statement to set a
limit of one client per interface. The interface-client-limit statement with a value of 1 retains the existing client and rejects
any new client connections. The client-discover-match incoming-interface statement deletes the existing client and allows a new client to
connect.
DHCP Local Server Handling of Client Information Request Messages
DHCP clients that already have externally provided addresses may solicit further configuration information from a DHCP server by sending a DHCP inform or DHCPv6 information-request message that indicates what information is desired. These message types can be collectively referred to as information request messages. By default, DHCP local server and DHCPv6 local server ignore any DHCP information requests that they receive. You can override this default behavior to enable processing of these messages.
If you enable processing of information requests, DHCP local server responds to the client with a DHCP acknowledgment message that includes the requested information—if it is available. DHCPv6 local server responds in the same manner but uses a DHCP reply message. No subscriber management or DHCP-management is applied as a result of the DHCP information request message.
By default, DHCP relay and DHCP relay proxy automatically forward DHCP information request messages without modification if the messages are received on an interface configured for a DHCP server group. DHCP relay and relay proxy drop information request messages received on any other interfaces. You cannot disable this default DHCP relay and relay proxy behavior.
The information requested by these clients is typically configured
with the dhcp-attributes statement for an address pool
defined by the address-assignment pool pool-name statement at the [edit access] hierarchy level.
When you enable processing of DHCP information requests, you can optionally specify the name of the pool from which the local server retrieves the requested configuration information for the client. If you do not do specify a local pool, then the local server requests that AAA selects and returns only the name of the relevant pool.
PPP interfaces are not supported on EX Series switches.
When DHCPv6 is configured over PPP interfaces, the PPP RADIUS authentication data can be used to select the pool from which the response information is taken. Additionally other RADIUS attributes can also be inserted into the DHCPv6 reply message. If an overlap exists between RADIUS attributes and local pool attributes, the RADIUS values are used instead of the local configuration data. If no RADIUS information is received from the underlying PPP interface, then the behavior is the same as described previously for non-PPP interfaces.
Enabling Processing of Client Information Requests
Configure one or more local address pools if you want to use a local pool rather than one provided by AAA. See DHCPv6 Address-Assignment Pools. For processing information request messages, the address configuration is not necessary. For DHCP local server, you must specify the IPv4 family; for DHCPv6 local server, you must specify the IPv6 family.
See Configuring DHCP Client-Specific Attributes Applied When Clients Obtain an Address for details about how to configure the information sought by clients that send information request messages.
By default, DHCP local server and DHCPv6 local server do not respond to information request (DHCP inform and DHCPv6 information-request) messages from the client. You can enable DHCP local server and DHCPv6 local server to process these messages and respond to them with an acknowledgment (ack or reply message, respectively) and the requested information.
DHCP relay agent automatically forwards the information request messages without modification to the configured server group by means of the interfaces configured for the respective server group. The messages are dropped if they are received on an unconfigured interface. DHCP relay proxy also supports forwarding these messages. You cannot disable forwarding of the information request messages.
To enable processing of DHCP client information request messages:
Sending Release Messages When Clients Are Deleted
By default, when DHCP relay and relay proxy delete a client, they do not send a release message to the DHCP server. You can override the default behavior and configure DHCP relay and relay proxy to send a release message whenever they delete a client. The release message sent by DHCP relay and relay proxy includes option 82 information.
You must include the send-release-on-delete statement to configure DHCP relay and relay proxy to send the release
message when the client-discover-match statement is included.
You can use the [edit forwarding-options dhcp-relay dhcpv6] hierarchy level to override the default behavior for DHCPv6 relay
agent.
To send a release message: