Ip helper address on 2960 with C2960S-UNIVERSALK9-M image

[Fabri_Fabri]

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,

[Georg Pauwen]

Hello,

what is the output of 'show sdm prefer' on your switch ?

[Fabri_Fabri]

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 ?

[paul driver]

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

[Fabri_Fabri]

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,

[paul driver · Accepted solution]

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

[Fabri_Fabri]

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,

[paul driver]

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

[Fabri_Fabri]

Thanks Paul ! It works like a charm !

[paul driver]

Hello

where are you trying to apply the helper?

Fyi - it needs to be on a routed interface

res

paul

[Fabri_Fabri]

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

[eaprice2008 · Accepted solution]

Here is an example of a vlan that is configured with an IP helper address.

[paul driver]

Hello

can you ping that dhcp server -Is it reachable?

res

paul

[Fabri_Fabri]

Hi Paul,

yes I can ping that DHCP server from the Switch and from the clients too.

[Fabri_Fabri]

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,