12-30-2016 07:37 AM - edited 03-08-2019 08:45 AM
Hi,
I need to configure ip helper address to use a dhcp server that is on a different vlan than clients.
Our switches have C2960S-UNIVERSALK9-M image but it seems that ip helper is not there.
Can you please help ?
Thanks,
Solved! Go to Solution.
12-30-2016 10:41 AM
Here is an example of a vlan that is configured with an IP helper address.
01-02-2017 05:45 AM
Hello
Just add the host IP address of the server
interface Vlan_Myvlan
ip address 10.0.0.1 255.255.0.0
ip helper-address 192.168.0.10
res
Paul
12-30-2016 08:49 AM
Hello,
what is the output of 'show sdm prefer' on your switch ?
01-02-2017 12:36 AM
Here you are:
switch48-01#show sdm prefer
The current template is "default" template.
The selected template optimizes the resources in
the switch to support this level of features for
0 routed interfaces and 255 VLANs.
number of unicast mac addresses: 8K
number of IPv4 IGMP groups + multicast routes: 0.25K
number of IPv4 unicast routes: 8.375k
number of directly-connected IPv4 hosts: 8K
number of indirect IPv4 routes: 0.375k
number of IPv6 multicast groups: 0.25K
number of directly-connected IPv6 addresses: 0.25K
number of indirect IPv6 unicast routes: 0
number of IPv4 policy based routing aces: 0
number of IPv4/MAC qos aces: 0.375k
number of IPv4/MAC security aces: 0.375k
number of IPv6 policy based routing aces: 0
number of IPv6 qos aces: 60
number of IPv6 security aces: 0.125k
Does that mean that I can't use that feature ?
Thanks for help ?
01-02-2017 05:14 AM
Hello
The helper address is used to direct dhcp requests to a dhcp server which is not local to the client ( not on the same subnet)
So yes you can apply the relay to the svi of the 2960 switch but the switch needs to able to reach the adddress you assign.
If the server is reachable then it's also possible access control lists are prohibiting the UDP access between client and server
res
Paul
01-02-2017 05:38 AM
Hi Paul,
yes the dhcp address is reachable from the switch.
I don't understand how to configure ip helper in my scenario: my 48 ports switches have only one port with an ip address configured and it is the one I use to administer them.
Example:
interface Vlan_Myvlan
ip address 10.0.0.1 255.255.0.0
!
ip default-gateway 10.0.0.2
ip http server
ip http secure-server
Do I have to configure this way that interface, to make ip helper work ?:
interface Vlan_Myvlan
ip address 10.0.0.1 255.255.0.0
ip helper-address 192.168.0.10 <subnet too?>
!
ip default-gateway 10.0.0.2
ip http server
ip http secure-server
and that's it ? From that moment on, clients could broadcast and get replies from 192.168.0.10 dhcp server too ?
Thanks,
01-02-2017 05:45 AM
Hello
Just add the host IP address of the server
interface Vlan_Myvlan
ip address 10.0.0.1 255.255.0.0
ip helper-address 192.168.0.10
res
Paul
01-02-2017 06:07 AM
Just tried to add ip helper-address 192.168.0.10, connected a client but no address is received.
Can you please suggest some commands to diagnose ?
Thanks,
01-02-2017 06:55 AM
Hello
Do you have dhcp snooping enabled on the switch or do have option 82 enabled?
If you have snooping enabled then the uplink port of the switch needs to be trusted or snooping disabled.
int x/x
description switch uplink
ip dhcp snooping trust
Dhcp option 82 provides an additional layer of security when assigning dhcp addressing which can negate allocation, if enabled try the following:
ip dhcp relay information trust-all
or
ip dhcp relay information policy keep
res
Paul
01-02-2017 07:46 AM
Thanks Paul ! It works like a charm !
12-30-2016 10:23 AM
Hello
where are you trying to apply the helper?
Fyi - it needs to be on a routed interface
res
paul
01-02-2017 12:47 AM
I don't know where it has to be applied.
I have many 2960 with a single ip address assigned to an interface that is the one I put as destination address when I connetc to them through telnet. All other interfaces have only these attributes:
interface GigabitEthernet1/0/38
description <my_descr>
switchport access vlan <vlan_id>
What can I do in this scenario ?
Thanks
12-30-2016 10:41 AM
12-30-2016 10:58 AM
Hello
can you ping that dhcp server -Is it reachable?
res
paul
01-02-2017 05:28 AM
Hi Paul,
yes I can ping that DHCP server from the Switch and from the clients too.
01-02-2017 12:40 AM
The only interface with an ip address I have is the one I connect to to administer the switch via telnet/ssh. Do I need to apply ip helper-address there ?
Here is conf:
interface VlanC1
ip address 10.50.1.1 255.255.0.0
!
all other interfaces have no ip address.
Thanks,
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