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Configuring HP 2910al's for multiple vlans and DHCP relay

Posted on 2013-01-25
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Last Modified: 2013-02-01
I am configuring 2  X 2910al's with a 4GB trunk between them and 11 VLans.  Everything is working as it should except DHCP.  Workstations on the management/primary vlan pick up their DHCP addresses just fine but all other vlans fail.  How do I configure a seperate gateway IP address (I'm assuming that the helper IP is the vlan's gateway) for each of the other vlans.  Everytime I try and add an IP for a vlan (using the gui) other than the primary, I lose my connection to the switch and I find that the primary IP has been changed.

Please help!

Thank you.
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Question by:Mark Wipf
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by:agonza07
Comment Utility
I think you are confusing your terms here.

IP Helper allows you to specify the DHCP server address used for that VLAN.

For example, let's say ur VLAN 1 has a DHCP server 192.168.1.100. Clients on VLAN 1 will broadcast their DHCP request and since the server is on the same VLAN it will respond with a DHCP address for the client.

Now, a client on VLAN 2 will broadcast the request, but since a broadcast is only local to a VLAN the dhcp server on VLAN 1 will not respond.

That's where IP Helper comes in. On the VLAN 2 gateway configure IP Helper of 192.168.1.100, and it will forward the DHCP request to the DHCP server. The ip address of the gateway forwarding that request will determine what DHCP pool the DHCp server will respond with.

Hope it sorta makes sense.

Let me check out the guide on those switches and I'll respond back on how to set gateway IPs... Or maybe someone else will respond on that one before me.
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Expert Comment

by:agonza07
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Page 7-4 shows you how to add IP addresses to different VLANs.
http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c03390015/c03390015.pdf

let me know how it works out for you. If it doesn't please send over some screenshots as I dont have access to a 2901al myself.

On page 5-56 it shows you the command line way to do an IP address and add the IP Helper command on VLAN 2. Check that out too.
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Author Comment

by:Mark Wipf
Comment Utility
Here's my running configs:

J9148A Configuration Editor; Created on release #W.15.08.0012
; Ver #02:11.05:16
hostname "UserSwStack1"
module 1 type j9148a
trunk 41-44 trk1 lacp
stack commander "UserSwStack"
stack member 1 mac-address 0026f1-4cc940
ip default-gateway 10.0.100.1
snmp-server community "public" unrestricted
spanning-tree
spanning-tree Trk1 priority 4
vlan 1
   name "DEFAULT_VLAN"
   no untagged 3-40
   untagged 45-48
   tagged 1-2,Trk1
   no ip address
   exit
vlan 10
   name "Egress"
   untagged 10
   tagged 1-2,Trk1
   no ip address
   exit
vlan 100
   name "LAN_IT"
   untagged 3-9
   tagged 1-2,Trk1
   ip address 10.0.100.15 255.255.255.0
   exit
vlan 110
   name "LAN_110"
   untagged 11-17
   tagged 1-2,Trk1
   no ip address
   exit
vlan 120
   name "LAN_120"
   untagged 18-23
   tagged 1-2,Trk1
   no ip address
   exit
vlan 130
   name "LAN_130"
   untagged 24-28
   tagged 1-2,Trk1
   no ip address
   exit
vlan 140
   name "LAN_140"
   tagged 1-2,Trk1
   no ip address
   exit
vlan 150
   name "LAN_150"
   untagged 29-33
   tagged 1-2,Trk1
   no ip address
   exit
vlan 160
   name "LAN_160"
   tagged 1-2,Trk1
   no ip address
   exit
vlan 170
   name "LAN_170"
   tagged 1-2,Trk1
   no ip address
   exit
vlan 180
   name "LAN_180"
   untagged 34-35
   tagged 1-2,Trk1
   no ip address
   exit
vlan 190
   name "LAN_190"
   untagged 36-40
   tagged 1-2,Trk1
   no ip address
   exit
primary-vlan 100
management-vlan 100
no autorun
password manager



I'm thinking that instead of no ip address for each non-primary vlan I should assign an ip with in the vlan such as: 'ip address 10.0.140.15 255.255.255.0' for vlan 140.  What I don't understand is that in our current switching environment, we have Dell 6248's and there is no helper ips or configured ip for each vlan and everything works beautifully.  The uplink to the server switch stack where the DHCP server is located passes through a firewall that acts as a policy engine and takes the tagged traffic and passes it through the correct gateway so the DHCP server knows what to respond with.  I hope this helps clarifies the issue.

Your assistance is appreciated.
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Accepted Solution

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schaps earned 250 total points
Comment Utility
You're missing an important line in your config:

ip routing

You just need it on the one 2910al, and you should also define an IP on each VLAN on that switch. Each VLAN should also have the line:

ip helper-address 10.0.100.x  <--address of your DHCP server

and then the switch will know that DHCP requests on those vlans with that line should be forwarded to VLAN 100.
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Author Comment

by:Mark Wipf
Comment Utility
Thank you.  I will give that a try and report back.
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Author Closing Comment

by:Mark Wipf
Comment Utility
ip routing was all that was missing.  Once I added it to Switch #1, dhcp started working on all vlans and subnets.
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