secondary ip helper-address for interface vlan or subinterface

[kapydan88]

Hello for everybody.

Is it possible to add secondary ip helper-address for interface vlan or subinterface (like ip address secondary).

I tried with different ios vesrion with catalys 9300, isr 4k and 2g - but all unsuccesfull.

OF-00-SW-03#sh run int vlan190
Building configuration...

Current configuration : 114 bytes
!
interface Vlan190
description VoIP
ip address 10.193.190.1 255.255.255.0
ip helper-address 10.194.10.55
end

OF-00-SW-03#conf t
Enter configuration commands, one per line. End with CNTL/Z.
OF-00-SW-03(config)#int vlan190
OF-00-SW-03(config-if)#ip helper
OF-00-SW-03(config-if)#ip helper-address 10.191.10.15 ?
<cr> <cr>

OF-00-SW-03(config-if)#ip helper-address 10.191.10.15 se
OF-00-SW-03(config-if)#ip helper-address 10.191.10.15 se?
% Unrecognized command
OF-00-SW-03(config-if)#ip helper-address ?
A.B.C.D IP destination address
global Helper-address is global
vrf VRF name for helper-address (if different from interface VRF)

OF-00-SW-03(config-if)#ip helper-address

[marce1000]

 - FYI : https://community.cisco.com/t5/switching/possible-to-have-multiple-quot-ip-helper-address-quot-entries/td-p/637817

 M.

[marce1000]

 - Actually , you don't need the secondary flag, just add the needed addresses.

 M.

[Richard Burts]

In reading the original post I am not clear exactly what is the intent. If the intent is secondary in the sense that it would be used only if the primary does not work then this is not possible. If the intent is secondary in the sense of an additional server to send requests to then the response from @marce1000 is spot on.

[Joseph W. Doherty]

Rick asks a great question, i.e. what's your intent with "secondary".  As Rick notes, adding additional DHCP servers can be done, but not as backups, simply by using the same statement format, as noted by Marce.

BTW, all IP helper addresses are treated "equally", so if you're wondering which DHCP will actually provide the IP, generally which ever one responds faster (assuming it has an IP it can provide).

[kapydan88]

Our current situation. There is cisco l3 switch with external dhcp pool for voip devices and int vlan for this devices.

...

interface Vlan190
description VoIP
ip address 10.193.190.1 255.255.255.0 - this is current network for voip
ip helper-address 10.194.10.55 - this is current win server with dhcp pool
end

What are we going to do - change current adressing scheme and ip address of external win server with dhcp pool

...

interface Vlan190
description VoIP
ip address 10.17.193.1 255.255.255.0 - this is new network for voip
ip helper-address 10.191.10.15 - this is new win server with dhcp pool
end

If i understand correctly, for this purpose we need to configure new ip as secondary for this int vlan and add second ip helper-address

interface Vlan190
description VoIP
ip address 10.193.190.1 255.255.255.0

ip address 10.17.193.1 255.255.255.0 secondary

ip helper-address 10.194.10.55

ip helper-address 10.191.10.15
end

[Joseph W. Doherty · Accepted solution]

Generally (or by default) Cisco helper IPs will not forward DHCP requests for secondary IPs.

For a migration like yours, what's often done is the new network is assigned the primary while the previous network is assigned as the secondary.  Then as hosts renew their DHCP, they will obtain the network to use.  Once all the hosts have been migrated off the previous network, it can be removed as a secondary IP.