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, due to which clients stuck in renew state.

The following similar configuration will be in place, i.e, multiple dhcp server groups will be configured under the server-group:

[edit forwarding-options dhcp-relay]

# Server groups
user@host# set server-group Default 203.0.113.1
user@host# set server-group Campus-A 198.51.100.100
user@host# set server-group Campus-A 198.51.100.101
user@host# set server-group Campus-B 198.51.100.55
user@host# set server-group Campus-B 198.51.100.56

# Default server group applied globally.

user@host# set active-server-group Default

# Interface groups with application of active server groups

user@host# set group Campus-A-v10-DHCP-RELAY interface ge-1/1/0.1
user@host# set group Campus-A-v10-DHCP-RELAY interface ge-1/1/0.2
user@host# set group Campus-A-v10-DHCP-RELAY interface ge-1/1/0.3
user@host# set group Campus-A-v10-DHCP-RELAY active-server-group Campus-A
user@host# set group Campus-B-v200-DHCP-RELAY interface ge-2/2/0.4
user@host# set group Campus-B-v200-DHCP-RELAY interface ge-2/2/0.5
user@host# set group Campus-B-v200-DHCP-RELAY interface ge-2/1/0.6
user@host# set group Campus-B-v200-DHCP-RELAY active-server-group Campus-B

To enable ACK forwarding from any server in the active server group, the below command helps in enabling the ACK forwarding globally.

Enable forwarding for the active server group:

[edit forwarding-options dhcp-relay active-server-group]
user@host# set allow-server-change

To enable ACK forwarding for a particular server-group:

#set forwarding-options dhcp-relay group <group-name> active-server-group allow-server-change

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DHCP ACKs are still being dropped in some cases in an MC-LAG environment even if with "allow-server-change" enabled. Engineering team has adjusted corresponding codes to fix the issue as per PR#1833148.

Fixed versions contain 21.4R3-S10, 22.4R3-S6, 24.2R2, 24.4R1 and 25.1R1.

2025-01-24: Update KB with PR number

2024-03-25: Article validated
2024-02-24 : Article Created

DHCP Active Server Groups | Junos OS | Juniper Networks