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DHCP Active Server Groups

 

You can apply a common DHCP or DHCPv6 relay configuration to a set of DHCP server IP addresses configured as a server group. For this, you must configure a group of DHCP server addresses, and apply them as an active server group. For more information, read this topic.

Configuring Active Server Groups to Apply a Common DHCP Relay Agent Configuration to Named Server Groups

You can apply a common DHCP or DHCPv6 relay configuration to a set of IP addresses configured as a server group. An active server group is sometimes referred to as a trusted group of servers.

You can configure active server groups globally or at the group level (configured with the group. When you apply the active server group at the group level, it overrides a global active server group configuration.

To configure a group of DHCP server addresses and apply them as an active server group:

  1. Specify the name of the server group.
    • For DHCPv4 servers:

    • For DHCPv6 servers:

  2. Add the IP addresses of the DHCP servers belonging to the group.
    Note

    Starting in Junos OS Release 18.4R1, up to 32 server IP addresses are supported per DHCPv4 server group. In earlier releases, a maximum of 5 server IP addresses are supported for DHCPv4 servers. Configuring more than the maximum number of server addresses results in a commit check failure.

  3. Apply the server group as an active server group.
    • At global level (DHCPv4)

    • At group-level (DHCPv6)

    • At global level (DHCPv6)

    • At group-level (DHCPv6)

For example, you might want to direct certain DHCP client traffic to a DHCP server. You can configure an interface group for each set of clients, specifying the DHCP relay interfaces for the group. In each of these groups, you specify an active server group to which each client groups traffic is forwarded. After a DHCP server group is created and server IP addresses are added to the group, the device used as the DHCP relay agent can forward messages to specific servers.

  • Three groups of DHCP server addresses are configured, Default, Campus-A, and Campus-B.

  • The Default group is applied as the global active server group for the overall DHCP relay configuration.

  • The Campus-A server group is assigned as the active server group for interface group Campus-A-v10-DHCP-RELAY. DHCP traffic received on the interfaces in Campus-A-v10-DHCP-RELAY is forwarded to DHCP servers 198.51.100.100 and 198.51.100.101.

  • The Campus-B server group is assigned as the active server group for interface group Campus-B-v200-DHCP-RELAY. DHCP traffic received on the interfaces in Campus-B-v200-DHCP-RELAY is forwarded to DHCP servers 198.51.100.55 and 198.51.100.56.

  • All other DHCP traffic is forwarded to DHCP server 203.0.113.1.

Note the following:

  • In some configurations, servers in an active server group maintain redundant information about the DHCP clients. If the binding server later becomes inaccessible, the client is unable to renew the lease from that server. When the client attempts to rebind to a server, other servers in the group with the client information can reply with an ACK message. By default, instead of forwarding the ACK to the DHCP client, the relay agent drops any such ACKs that it receives from any server other than the binding server because the new server address does not match the expected server address in the DHCP client entry. Consequently the lease cannot be extended by any of the redundant servers.

  • Starting in Junos OS Release 16.2R1, you can enable a DHCPv4 relay agent to forward DHCP request (renew or rebind) ACKs from any server in the active server group (thus, any trusted server). The relay agent updates the client entry with the new server address. Because the servers in the group are expected to mirror the client information exactly, the lease option is expected to be the same as for the original server and the relay agent does not need to verify the lease option.

  • Starting in Junos OS Release 18.4R1, this capability is extended to allow a DHCP relay agent to forward DHCP information request (DHCPINFORM) ACK messages from any server in the active server group.

To enable ACK forwarding from any server in the active server group:

  • Enable forwarding for the active server group.

Release History Table
Release
Description
Starting in Junos OS Release 18.4R1
Starting in Junos OS Release 16.2R1