topic DHCP for subnet between two router in Switching

DHCP for subnet between two router

hydakyo — Fri, 08 Mar 2019 23:30:13 GMT

Help me!
DHCP for laptop bet R3 and R4 in subnet 5, R1 is DHCP source

Re: DHCP for subnet between two router

Ben Walters — Wed, 27 Jun 2018 15:49:28 GMT

You just need to use the ip helper-address <R1 ip> command.

That way when a request is seen on either R3 or R4 it will forward it to R1 to handle the request.

Re: DHCP for subnet between two router

Jean-Pierre de Villeres — Wed, 27 Jun 2018 16:29:08 GMT

Hi hydakyo,

Please find below sample config when R1 is DHCP server and Laptop will get IP 172.16.1.100.

R3 and R4 will use helper address of R1 10.255.1.1 to get the DHCP IP for the laptop. Please ensure no blocking and proper routing in intermediary devices.

R1#
int loopback 0
10.255.1.1

ip dhcp pool LAPTOP
host 172.16.1.100 255.255.255.0
default-router 172.16.1.3 172.16.1.4
ip dhcp excluded-address 172.16.1.3
ip dhcp excluded-address 172.16.1.4

R3#
int fa0/1
ip address 172.16.1.3 255.255.255.0
ip helper-address 10.255.1.1

R4#
int fa0/1
ip address 172.16.1.4 255.255.255.0
ip helper-address 10.255.1.1

LAPTOP set to get DHCP IP 172.16.1.100 255.255.255.0

Re: DHCP for subnet between two router

hydakyo — Wed, 27 Jun 2018 17:14:25 GMT

SW1-> SW2 ->R3 is VLan 10 , SW1-> SW3-> R4 is Vlan20
I was use ip helper-address <r1 ip>, but it's fail ,when i remove connection F0/2 of Switch 4 and F0/1 of R4 and add ip helper-address in F0/0.10 of R2 (Vlan 10) it successfull , when i connect SW4 and R4 it fail 😞 , so sad

Re: DHCP for subnet between two router

hydakyo — Wed, 27 Jun 2018 17:39:16 GMT

ip dhcp pool LAPTOP
host 172.16.1.100 255.255.255.0
default-router 172.16.1.3 172.16.1.4
ip dhcp excluded-address 172.16.1.3
ip dhcp excluded-address 172.16.1.4

can't enter <default-router 172.16.1.3 172.16.1.4> in packet tracer ?

Re: DHCP for subnet between two router

balaji.bandi — Thu, 28 Jun 2018 06:41:54 GMT

But in device the command still valid and works. Try only "default-router 172.16.1.3" see how it goes