How to change the DHCP relay/helper on H3C S5800 Series switch ?

[Server-Support]

Can anyone here assist me in what command should I use to change the DHCP relay (helper) setting on H3C S5800 Series switch on the below VLAN:

10.1.2.0/24 - Workstation level 1
10.1.3.0/24 - Wireless LAN level 1
10.1.4.0/24 - Workstation level 2
10.1.5.0/24 - Wireless LAN level 2

Old DHCP server IP: 10.1.3.2
New DHCP server IP: 10.1.3.196

I've gone through: https://h20566.www2.hpe.com/portal/site/hpsc/template.PAGE/public/kb/search/?sp4ts.oid=4179963&spf_p.tpst=kbSearchMain&spf_p.pbp_kbSearchMain_myAction=withResults&javax.portlet.begCacheTok=com.vignette.cachetoken&javax.portlet.endCacheTok=com.vignette.cachetoken#searchResultHead

but still not sure how to do it and what could be the impact for the production network traffic flow.

Does it requires switch restart or reboot ?

[EricAtHP · Accepted solution]

Assuming that DHCP is/was working with the old server, all you need to do is add the new server to the existing relay server group:

From the system prompt:

[HP-5800]dhcp relay server-group <your group id> ip 10.1.3.196

Then remove the old server:

[HP-5800]undo dhcp relay server-group <your group id> ip 10.1.3.2

For a configuration from scratch:

[HP-5800]dhcp enable

[HP-5800]dhcp relay server-group 1 ip 10.1.3.196

[HP-5800]interface vlan 1

[HP-5800-Vlan-interface1]dhcp relay server-select 1

[HP-5800-Vlan-interface1]dhcp select relay

Use the "display dhcp relay all" command to view the existing configuration.

[Server-Support]

Hi Eric,

Thanks for the reply. So is there any connectivity outage or switch reboot after updating the dhcp relay ?

[EricAtHP]

No reboot required and no connectivity loss.

The way dhcp realy works is that when the switch receives a DHCP broadcast, it turns it into a unicast and forwards it to all of the configured dhcp servers. All of the servers reply and the switch forwards all of the replies to the requesting client. The client then chooses one of the responses. Usually the one that offers the address the client is already using or the first response if it is the first time on the network.

That is why I recommended configuring the new server first and then removing the old server. You could add a step and test with a new client. If the client gets an address from the new server, then you are good to go. You may have to use a packet capture utility on the client to validate that it is getting offers from both dhcp servers. 

[Server-Support]

Many thanks for assisting me in this old legacy H3C S5800 Series switch. I didn't realize it that I can find someone who can assist me in this forum.

[Server-Support]

Hi Eric,

May I know hwere to get the value of GroupID ?

[EricAtHP]

It would have been configured by whoever did the initial config. Do a "display current" to view the config and look for it in the output.

[manuel.bitzi]

Hi Albert

The 5800 is a state of the art switch and still under development. There will be bigger announcements in the next months. Don't worry you are on the right track.

br

Manuel

[Server-Support]

Many thanks for the reply Manuel.

It is hard to find a Good online resource for H3C apart from this great forum. Because most environment uses Cisco or Juniper switches but not H3C.

So it seems that HP Procurve switch and H3C is almost similar in terms of command syntax.

[manuel.bitzi]

Hi Albert

The H3C 5800 is the same device as the HP 5800. You can change the brand if you want. Here you will find a lot of ressources:

http://h20565.www2.hp.com/portal/site/hpsc/public/psi/home/?sp4ts.oid=4177501&ac.admitted=1429510758745.876444892.492883150#manuals

br

Manuel