Cisco Management Vlan and IP helper address command

[mahesh18]

                   Hi Everyone.

On Layer 2 Network we have Access Layer Switch with

Vlan 10 IP 192.168.10.50    Management interface

Gw 192.168.10.254

then on core switch we have vlan 10 config as below

interface Vlan10

ip vrf forwarding Networks10

ip address 192.168.10.254 255.255.255.0

ip helper-address 172.31.119.1

I need to add one more switch in the network and it will have management vlan as 10.

Need to know use of ip helper-address command under int vlan 10?

does it is used to assign IP to new switches when we add them in network under vlan 10?

Thanks

Mahesh

[John Blakley · Accepted solution]

Mahesh,

DHCP broadcasts a packet to find a dhcp server that will respond to its request. When you have vlans configured, broadcasts will not leave the vlan and therefore wouldn't be able to find a dhcp server on another vlan. The 'ip helper-address' command converts the broadcast packet to a unicast packet and sends it directly to the ip address that you have configured on the helper address. This way, you can "break out" of the vlan in order to be able to use a dhcp server on a different vlan.

HTH,
John

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[mahesh18]

Hi John,

Thanks for reply.

So my understanding is that all the Management Interface IP in different switches are getting IP address by  the dhcp server right?

Seems we have assigned static IP to all vlan 10 interfaces in different switches as layer 2 switch vlan 10 config is

int vlan 10

ip address 192.168.10.X  right ?

Also if we add new switch to network then  we have to know which IP is free in the dhcp server pool?

thanks

mahesh

[John Blakley · Accepted solution]

Mahesh,

Yes, in this case your interface is configured like the following:

interface Vlan10

ip vrf forwarding Networks10

ip address 192.168.10.254 255.255.255.0

ip helper-address 172.31.119.1

The ip addresses in this vlan need to be in the 192.168.10.0/24 subnet. You'll have a dhcp scope of 192.168.10.0/24 on your dhcp server at address 172.31.119.1. If the devices in vlan 10 are set statically, the helper address is of no use in the dhcp configuration area because your hosts wouldn't be broadcasting for an address =).

HTH,
John

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[mahesh18]

Hi John,

On more thing so in this case we have used ip helper to send management traffic to next device right as it is not used for DHCP ?

Thanks

Mahesh

[John Blakley · Accepted solution]

It depends on what 172.31.119.1 does. The helper address is used for udp broadcast packets, so it technically can be used for things other than dhcp. It's hard for me to tell you what you'd be using it for because as it stands now it looks like it's being used for dhcp (99% of the time is what it's used for).

HTH,
John

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[mahesh18]

Hi John,

Thanks for answering all the questions.

I will check with some one in my company to know purpose of ip heler under vlan.

Regards

Mahesh