Switching

set access address-assignment pool wifi family inet network 172.16.8.0/21
set access address-assignment pool wifi family inet range wifi low 172.16.8.120
set access address-assignment pool wifi family inet range wifi high 172.16.15.254
set access address-assignment pool wifi family inet dhcp-attributes maximum-lease-time 108000
set access address-assignment pool wifi family inet dhcp-attributes name-server 8.8.8.8
set access address-assignment pool wifi family inet dhcp-attributes name-server 49.45.0.4
set access address-assignment pool wifi family inet dhcp-attributes name-server 103.8.46.5
set access address-assignment pool wifi family inet dhcp-attributes router 172.16.8.1

You can confirm what the current setting is with this status command
show system services dhcp statistics

I think without setting any it should be 2 hours.

Do you have dhcp as an allowed service for the zone of the gateway interface?

set access address-assignment pool wifi family inet network 172.16.8.0/21
set access address-assignment pool wifi family inet range wifi low 172.16.8.120
set access address-assignment pool wifi family inet range wifi high 172.16.15.254
set access address-assignment pool wifi family inet dhcp-attributes maximum-lease-time 108000
set access address-assignment pool wifi family inet dhcp-attributes name-server 8.8.8.8
set access address-assignment pool wifi family inet dhcp-attributes name-server 49.45.0.4
set access address-assignment pool wifi family inet dhcp-attributes name-server 103.8.46.5
set access address-assignment pool wifi family inet dhcp-attributes router 172.16.8.1

This command is showing not valid on ex4650 and if this is by default two hours still ips are changing in few minutes after disconnect and connecting again.

admin@jklucoreswitch> show system services dhcp statistics
error: command is not valid on the ex4650-48y-8c

You can confirm what the current setting is with this status command
show system services dhcp statistics

I think without setting any it should be 2 hours.

Do you have dhcp as an allowed service for the zone of the gateway interface?

set access address-assignment pool wifi family inet network 172.16.8.0/21
set access address-assignment pool wifi family inet range wifi low 172.16.8.120
set access address-assignment pool wifi family inet range wifi high 172.16.15.254
set access address-assignment pool wifi family inet dhcp-attributes maximum-lease-time 108000
set access address-assignment pool wifi family inet dhcp-attributes name-server 8.8.8.8
set access address-assignment pool wifi family inet dhcp-attributes name-server 49.45.0.4
set access address-assignment pool wifi family inet dhcp-attributes name-server 103.8.46.5
set access address-assignment pool wifi family inet dhcp-attributes router 172.16.8.1

Sorry, I missed this was EX and not SRX.

Do you have the interface assigned in the system area?

set system services dhcp-local-server group-name interface interface-name

https://www.juniper.net/documentation/en_US/junos/topics/topic-map/dhcp-for-switching-devices.html#id-configuring-a-switch-as-a-dhcp-server-cli-procedure

This command is showing not valid on ex4650 and if this is by default two hours still ips are changing in few minutes after disconnect and connecting again.

admin@jklucoreswitch> show system services dhcp statistics
error: command is not valid on the ex4650-48y-8c

You can confirm what the current setting is with this status command
show system services dhcp statistics

I think without setting any it should be 2 hours.

Do you have dhcp as an allowed service for the zone of the gateway interface?

set access address-assignment pool wifi family inet network 172.16.8.0/21
set access address-assignment pool wifi family inet range wifi low 172.16.8.120
set access address-assignment pool wifi family inet range wifi high 172.16.15.254
set access address-assignment pool wifi family inet dhcp-attributes maximum-lease-time 108000
set access address-assignment pool wifi family inet dhcp-attributes name-server 8.8.8.8
set access address-assignment pool wifi family inet dhcp-attributes name-server 49.45.0.4
set access address-assignment pool wifi family inet dhcp-attributes name-server 103.8.46.5
set access address-assignment pool wifi family inet dhcp-attributes router 172.16.8.1

i have assigned the interface on irb and assigned ip addresses to irb interfaces and made them l3 interface. is there any issue with server identifier command. i have not assigned the server identifier command.

set system services dhcp-local-server group IT interface irb.101
set system services dhcp-local-server group admin interface irb.102
set system services dhcp-local-server group faculty interface irb.105
set system services dhcp-local-server group event interface irb.108
set system services dhcp-local-server group residence interface irb.110
set system services dhcp-local-server group wifi interface irb.104
set system services dhcp-local-server group hostel interface irb.109
set interfaces irb unit 100 family inet address 172.16.1.2/25
set interfaces irb unit 101 family inet address 172.16.3.1/24
set interfaces irb unit 102 family inet address 172.16.2.1/24
set interfaces irb unit 103 family inet address 172.16.4.1/22
set interfaces irb unit 104 family inet address 172.16.8.1/21
set interfaces irb unit 105 family inet address 172.16.25.1/24
set interfaces irb unit 106 family inet address 172.16.26.1/24
set interfaces irb unit 108 family inet address 172.16.32.1/20
set interfaces irb unit 109 family inet address 172.16.16.1/21
set interfaces irb unit 110 family inet address 172.16.48.1/21
set protocols router-advertisement interface irb.0
set vlans IT l3-interface irb.101
set vlans LAB l3-interface irb.103
set vlans admin l3-interface irb.102
set vlans cctv l3-interface irb.106
set vlans event l3-interface irb.108
set vlans faculty l3-interface irb.105
set vlans fire l3-interface irb.100
set vlans hostel l3-interface irb.109
set vlans residence l3-interface irb.110
set vlans wifi l3-interface irb.104

Sorry, I missed this was EX and not SRX.

Do you have the interface assigned in the system area?

set system services dhcp-local-server group-name interface interface-name

https://www.juniper.net/documentation/en_US/junos/topics/topic-map/dhcp-for-switching-devices.html#id-configuring-a-switch-as-a-dhcp-server-cli-procedure

This command is showing not valid on ex4650 and if this is by default two hours still ips are changing in few minutes after disconnect and connecting again.

admin@jklucoreswitch> show system services dhcp statistics
error: command is not valid on the ex4650-48y-8c

You can confirm what the current setting is with this status command
show system services dhcp statistics

I think without setting any it should be 2 hours.

Do you have dhcp as an allowed service for the zone of the gateway interface?

set access address-assignment pool wifi family inet network 172.16.8.0/21
set access address-assignment pool wifi family inet range wifi low 172.16.8.120
set access address-assignment pool wifi family inet range wifi high 172.16.15.254
set access address-assignment pool wifi family inet dhcp-attributes maximum-lease-time 108000
set access address-assignment pool wifi family inet dhcp-attributes name-server 8.8.8.8
set access address-assignment pool wifi family inet dhcp-attributes name-server 49.45.0.4
set access address-assignment pool wifi family inet dhcp-attributes name-server 103.8.46.5
set access address-assignment pool wifi family inet dhcp-attributes router 172.16.8.1

The configuration looks complete.  Do you have any firewall filters applied on the switch?  
If a protection filter is applied to the loopback or any fixed interface it might be interfering with the traffic.

If not, we need to see what the status is of the EX dhcp system

Confirm that all the settings configured are being activated
show system services dhcp pool

Also confirm that DHCP snooping, ARP inspection or Mac limiting are not enabled for port security.

If none of this applies, we need to turn on logging, called trace options in Junos.  This specifies a file and we can see all the transactions and errors and see why the leases are being changed so frequently.

Configure debug logging and commitset system processes dhcp-service traceoptions file dhcpLog set system processes dhcp-service traceoptions flag allshow logs after commitshow log dhcpLog

documentation
https://www.juniper.net/documentation/us/en/software/junos/subscriber-mgmt-sessions/topics/ref/statement/traceoptions-edit-system-processes-dhcp-service.html

i have assigned the interface on irb and assigned ip addresses to irb interfaces and made them l3 interface. is there any issue with server identifier command. i have not assigned the server identifier command.

set system services dhcp-local-server group IT interface irb.101
set system services dhcp-local-server group admin interface irb.102
set system services dhcp-local-server group faculty interface irb.105
set system services dhcp-local-server group event interface irb.108
set system services dhcp-local-server group residence interface irb.110
set system services dhcp-local-server group wifi interface irb.104
set system services dhcp-local-server group hostel interface irb.109
set interfaces irb unit 100 family inet address 172.16.1.2/25
set interfaces irb unit 101 family inet address 172.16.3.1/24
set interfaces irb unit 102 family inet address 172.16.2.1/24
set interfaces irb unit 103 family inet address 172.16.4.1/22
set interfaces irb unit 104 family inet address 172.16.8.1/21
set interfaces irb unit 105 family inet address 172.16.25.1/24
set interfaces irb unit 106 family inet address 172.16.26.1/24
set interfaces irb unit 108 family inet address 172.16.32.1/20
set interfaces irb unit 109 family inet address 172.16.16.1/21
set interfaces irb unit 110 family inet address 172.16.48.1/21
set protocols router-advertisement interface irb.0
set vlans IT l3-interface irb.101
set vlans LAB l3-interface irb.103
set vlans admin l3-interface irb.102
set vlans cctv l3-interface irb.106
set vlans event l3-interface irb.108
set vlans faculty l3-interface irb.105
set vlans fire l3-interface irb.100
set vlans hostel l3-interface irb.109
set vlans residence l3-interface irb.110
set vlans wifi l3-interface irb.104

Sorry, I missed this was EX and not SRX.

Do you have the interface assigned in the system area?

set system services dhcp-local-server group-name interface interface-name

https://www.juniper.net/documentation/en_US/junos/topics/topic-map/dhcp-for-switching-devices.html#id-configuring-a-switch-as-a-dhcp-server-cli-procedure

This command is showing not valid on ex4650 and if this is by default two hours still ips are changing in few minutes after disconnect and connecting again.

admin@jklucoreswitch> show system services dhcp statistics
error: command is not valid on the ex4650-48y-8c

You can confirm what the current setting is with this status command
show system services dhcp statistics

I think without setting any it should be 2 hours.

Do you have dhcp as an allowed service for the zone of the gateway interface?

set access address-assignment pool wifi family inet network 172.16.8.0/21
set access address-assignment pool wifi family inet range wifi low 172.16.8.120
set access address-assignment pool wifi family inet range wifi high 172.16.15.254
set access address-assignment pool wifi family inet dhcp-attributes maximum-lease-time 108000
set access address-assignment pool wifi family inet dhcp-attributes name-server 8.8.8.8
set access address-assignment pool wifi family inet dhcp-attributes name-server 49.45.0.4
set access address-assignment pool wifi family inet dhcp-attributes name-server 103.8.46.5
set access address-assignment pool wifi family inet dhcp-attributes router 172.16.8.1