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DHCP/DNS Issue

Posted on 2016-07-22
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Last Modified: 2016-07-29
I have a site that has about 3 workstations, and IPs were statically assigned (configured on workstation and on infoblox (DHCP and DNS records created).   There are two switches, a core and access switch, both are LAN base images and use the router (2821) to route.  There is only one VLAN (VLAN1).  I recently added ip-helper addresses on the router's interface for dhcp/dns (both services are working fine), but when I change the workstation from static to DHCP, it doesn't pick up an IP address.  


Ideas:
It''s statically assigned in infoblox and dns records are still there for this workstation.  Could this be causing the issue of the workstation not receiving a leased IP?

Should the ip helper be assigned on VLAN 1 on the switches? (A similar site has it on the router's interface)  

I will try to get a resource on site to connect to the switch with another laptop.


Router
interface GigabitEthernet0/0
 ip address 10.124.117.1 255.255.255.128
ip helper-address 57.2.2.19
 ip helper-address 57.2.2.5

 no ip redirects
 no ip proxy-arp
 ip flow ingress
 ip route-cache flow
 duplex full
 speed 100
 service-policy input REMARK-TO-ZERO

Core
interface Vlan1
 ip address 10.124.117.3 255.255.255.128
 no ip route-cache

ip default-gateway 10.124.117.1

interface GigabitEthernet0/1
 description Router Gi0/1
 switchport trunk allowed vlan 1
 switchport mode trunk
 media-type sfp
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Question by:PeraHoman
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6 Comments
 
LVL 17

Expert Comment

by:lruiz52
ID: 41725434
If you only have one vlan, you don't need the ip helper command. Check your windows or av firewall, it may be blocking DHCP.
0
 
LVL 57

Expert Comment

by:giltjr
ID: 41725476
What is the IP address of the DHCP server?
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LVL 57

Accepted Solution

by:
giltjr earned 2000 total points
ID: 41725889
If your DHCP servers were in VLAN1 (10.124.117.0/25) then lruiz52 would be correct that you don't need IP helper statements, but from the above configuration even though you only have 1 VLAN, your DHCP servers are not in that VLAN.

Looking at your configuration it appears that 57.2.2.19 and 57.2.2.6 are your DHCP servers.  Is that correct?  It is unusual to have DHCP servers for a private network with public IP addresses.

However, if they are your DHCP servers do they have a route back to your 10.124.117.0/25 network.

Since your router is on VLAN1 and assuming the DHCP devices are the two with the 57 addresses, then you only need the helper statements on your router, it will forward the bootp requests.

If, by chance the two hosts with the 57 addresses are NOT your DHCP servers, then I need to understand why you have ip helper statements pointing to them.  Have they always been there.  If not, then I fairly sure you need to remove them.
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Author Comment

by:PeraHoman
ID: 41725899
DHCP servers are not in VLAN 1 subnet, and those are the correct addresses, and they are routable (mpls network).  DHCP/DNS addresses are the 57.x.x.x IPs.
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LVL 57

Expert Comment

by:giltjr
ID: 41725966
Can you ping both of those addresses from a computer in VLAN1 when it has a static IP address?

If you can, then I would double check how DHCP is configured on the DHCP servers.  I know you said the DHCP is working, but obviously it is not for requests from VLAN1.  If it was working for VLAN1 then you would not be here asking questions about it.
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LVL 57

Expert Comment

by:giltjr
ID: 41735013
Thanks for accepting my answer, but could you document what specifically the problem was?  That will help others that have the same problem.
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