DHCP Problem

[dimigav1977]

Dear all,

I have a problem with DHCP. My router act as DHCP Client and Server together.

The Gi0/0.837 WAN connection configured as DHCP Client to obtain IP from ISP.

The Gi0/1.2 Data Network with IP scope 192.168.2.0/24 and Gi0/1.100 Voice Network with scope 192.168.100.0/24 assigned from DHCP Server pools.

If the DHCP pool created the Wan cannot obtain IP from WAN. If I delete the pools the WAN successfully obtain IP.

Why this?

configuration

ip dhcp pool LAN
network 192.168.2.0 255.255.255.0
default-router 192.168.2.1
dns-server 8.8.8.8 8.8.4.4

!

ip dhcp pool VOICE
network 192.168.100.0 255.255.255.0
default-router 192.168.100.1

!

interface GigabitEthernet0/0.837
description WAN
ip address dhcp
!
interface GigabitEthernet0/1.2
description LAN1
ip address 192.168.2.1 255.255.255.0

!

interface GigabitEthernet0/1.100
description VOICE1
ip address 192.168.100.1 255.255.255.0

[Giuseppe Larosa]

Hello @dimigav1977 ,

it is quite strange the router should be able to act as DHCP client on an interface and at the same time to act as DHCP server on other interfaces.

can you post

show version

and also the complete configuration of your router ( you can put it in a text file and attach it to the post)

Hope to help

Giuseppe

[dimigav1977]

The working configuration....

[dimigav1977]

Also the failed config..

[Giuseppe Larosa]

Hello @dimigav1977 ,

can you explain the following  taken from config-failed file:

interface GigabitEthernet0/0
no ip address
duplex auto
speed auto
!
interface GigabitEthernet0/0.835
encapsulation dot1Q 835
pppoe enable group global
pppoe-client dial-pool-number 1
!
interface GigabitEthernet0/0.836
encapsulation dot1Q 836
ip address dhcp
ip nat outside
ip virtual-reassembly in
!
interface GigabitEthernet0/0.837
encapsulation dot1Q 837
ip address dhcp
!
interface GigabitEthernet0/1
no ip address
>>>>>> shutdown
duplex auto
speed auto
!
interface GigabitEthernet0/1.2
encapsulation dot1Q 2
ip address 192.168.2.254 255.255.255.0
ip nat inside
ip virtual-reassembly in
!
interface GigabitEthernet0/1.100
encapsulation dot1Q 100
ip address 192.168.100.254 255.255.255.0
!

you have created three different subifs one using PPPoE and two using ip address DHCP what is your real exit interface to the internet ?

Also the internal main interface is shutdown .it shouldn't be otherwise all related subifs are not operational too.

Hope to help

Giuseppe

[dimigav1977]

The three sub-interfaces are one for DATA from provider .835, the other for VOIP .837 and the last for IPTV .836 but this is inactive because no service paid for that. The physical connection is a GPON FTTH. The provider uses PPPoE for DATA and DHCP with 10.x.x.x address for VOIP..So, the real interface for internet (Data & Voip) is the Gi0/0.

Gi0/1 is the internal interface for the local network.

[Giuseppe Larosa]

Hello @dimigav1977 ,

thanks now I understand better your environment

interface Dialer1
mtu 1492
ip address negotiated
ip nat outside
ip virtual-reassembly in
encapsulation ppp
shutdown
dialer pool 1
dialer-group 1
no cdp enable
ppp authentication chap pap callin
ppp chap hostname xxxxx@xxxx.gr
ppp chap password 7 <removed>
ppp pap sent-username xxxxx@xxxx.gr password 7 <removed>
!

This interface is able to take an IP from ISP via PPPoE and the only one affected is the subif for VOIP ?

Is this correct ?

Hope to help

Giuseppe

[dimigav1977]

I think you understand better my environment, but the affected sub-interface 837 (cannot obtain IP) does not depend of the Dialer1.

At the evening I can send you a working config with dialer1 (sub-if 835) and DHCP client (sub-if 837) together. Something else in the failed config drop the DHCP connection. Even with DHCP debug I can't find the problem.

[dimigav1977]

Finally, you are right. Dialer on 835 subif and DHCP on 837 cannot work together. I tested this yesterday and I saw that after release of the DCP address with dialer active never obtain again new or the same IP.

[Giuseppe Larosa]

Hello @dimigav1977 ,

I have asked to you about your configuration because I had seen different subinterfaces using different access methods PPPoE on VLAN 835 and IP over ethernet with DHCP on VLAN 837.

I would suggest to connect the ISP with a L2 switch using  a trunk port carrying VLAN 835 and 837 and then to move one subinterface to another physical interface on the router if there is one available and to try again in this way.

The two router interfaces would be connected to two other ports on the L2 switch.

Hope to help

Giuseppe

[dimigav1977]

So, you suggest the attached wiring solution..

[dimigav1977]

Dear Giuseppe,

I tried the following config on Cisco switch and then on the router.

Router has never been able to connect to any connection (Dialer or DHCP).

Maybe there is a mistake on switch config.

My physical connections are the followings:

ISP -> Switch1 Gi0/1

Switch1 Gi0/2 -> Router Gi0/0

Switch1 Gi0/3 -> Router Gi0/1

Thanks

[Georg Pauwen]

Hello,

what if you disable the DHCP service:

837(config)#no service dhcp

which should allow the WAN interface to get an IP address, and then re-enable the service ?

837(config)#service dhcp

[dimigav1977]

No difference.

Even with no service dhcp command the WAN cannot obtain IP from ISP.

[paul driver]

Hello
This rtr is a PPOE client correct ?- can you confirm., If so you would require a logical dialler interface like- 

interface Dialer1
ip address negotiated 
encapsulation ppp
dialer pool 1 
ppp authentication chap
ppp chap hostname xxx