DHCP, vlans and a L2 switch (HP2520G)

[ElsM]

Hi All,

I'm preparing to get a voice subnet / vlan configured on our network that for the moment has all devices connected to the default vlan (1).

material:

3 switches HP2520G = SW A and HP2530G = SW B and HP2848 =SW C
1 Cisco router (managed by an external party)

iphones

Acutal DHCP range 10.32.10.x/24; dhcp setup on a windows DC (10.32.10.3). (don't want to give the DC 2 ip-adresses even if it has 2 seperate NICS).
DHCP server on vlan1 untagged port on SW C ..

I would like to introduce Vlan 11, 10.32.11.0/24 - all devices connected on a HP25xx switch.

For an iphone (Avaya 9631G) i would tag the switchport connected to the phone for vlan11 and untagged for vlan1. A "trunk" between SW C and SW A en between SW C and SW B for vlan 11 and vlan 1. Let's assume i put the phone on SW A.

I was thinking to be using the ip-helper command to get the ip-adresses assign to vlan11, but i found out that the HP2520G switches we have on site do not support DHCP-relay.
Can somebody confirm that this is indeed the case ?

If so,
If I Create a second scope for 10.32.11.x/24 on the DHCP server (10.32.10.3), don't change anything else for this server and his port, will my iphone get an ip-adress in the 10.32.11.x range if i assign the 10.32.11.1/24 adress to vlan11 on switch A ?

Or say I will connect a computer (with nic in vlan 11) to the port first and not my iphone, to see if it gets a 10.32.11.x address ?

Kind Regards,

Els

[ElsM]

since DHCP starts with a broadcast, my PC in vlan 11 will never get an ip-adress if in our 3rd party router doesn't forward the request.
Correct me if I am wrong here.

I my computer nic has no vlan associated, what address would he get ?
(and this i can test when i start configuring it)

My backup plans are:
adding a DHCP server in vlan 11 (the VoIp phone central) our ask to have a DHCP relay on the router.

[RobertDobre · Accepted solution]

Hi Els, 

The helper address function works only on a L3 device (switch or router) that is performing the inter-vlan routing for your vlans. The ip helper configuration is done on the new vlan interface (L3 interface).  

As your plan is to create an additional vlan for voice (vlan 11) you also need to create another scope on the DHCP anyway for the new range. Keep in mind that the DHCP request is a broadcast message and only L3 interfaces configured with DHCP relay will forward the request as unicast to the DHCP server, otherwise the request is not reaching the server located on another vlan. 

Looking to your switches type, you have 2 options:

1. Use the  Provision software switch HP2848 to perform your intervlan routing, connect all the other switches to them using tagged vlans  on one side (Provision side) and trunk on the other side (Comware side), choose another vlan for router connection and assign another IP range to it. This will imply a change on the router also (only the IP address on the router interface). If you would like to avoid changing the router configuration, you can allocate the existing IP address range only for router connexion and create 2 new IP ranges for data and voice (including 2 new DHCP scopes). You can then configure the ip helper command on vlan 11  making sure you have access to the DHCP server from vlan 11.

2. Use your switches only as L2 swithes, make the existing link to the router a tagged (commware trunk) link  permit the new vlan on the link and ask the router admins to create 2 subinterfaces on the link to your switch (router on a stick), one for data and another one for voice. Then on the router voice vlan subinterface they will configure the dhcp relay for you.

Hope that helps, 

Robert

[ElsM]

Hi Robert,
thanks for the reply. i will need to reread your point 1 in order to grasp what you are saying :-). No worries, I will get there.

[ElsM]

Hi Robert, would you also shed your light upon this topic:

Confused about DHCP to obtain IP Adress topic in HP2520 G Advanced traffic management guide