DHCP Failover configuration.

[vamsi harish]

Dear Team,

     As per the requirement i've created 10 VLANs. and also configued HSRP for failover. Out of 10 Vlans we have configured DHCP for 2 VLANs in the active switch.

My concerns:

1. Do we have to configure DHCP in standby switch too?

2. If yes, is there any Active and Standby concept for DHCP?

Switch Model:

WS-C4506-E

Regards,

Vamsi Harish.T.

vamsiharish@gmail.com

[Mohamed Sobair · Accepted solution]

Hi,

The Host sends a DHCP broadcast request message to the Active Switch whenever it needs to assign and get an ip address based on the DHCP release confguration on the DHCP server or when the Hosts Got shut down. So yes there is a Concept here, the Concept is that the Active Switch will always get the DHCP request from the HOST. However, if the Active Switch fails, The HOSTS will be sending the DHCP broadcast request messages to the Standby Switch which becomes the Active one now, and if there is no DHCP service configured on the Standby Switch, then Hosts will fail to assign/Got an IP addresses. This Means both Active & Standby Switches should be configured with DHCP.

HTH

Mohamed

[vamsi harish]

Dear Mohamed Sobair,

Thansk for your help. I've done the configuration according to your suggestion and it's working fine.

Regards,

Vamsi Harish.T.

vamsiharish@gmail.com

[devils_advocate]

Read my message above, it may work initially but I think you will eventually end up with duplicates.

[Network TSC Vuitton]

Hi Sobair,

I am having the same configuration as you explained. I have two core switches and I have the same DHCP pool in two core switches.

But everyday there are few IP conflicts. I need to clear it every day. Issue is gone when I remove DHCP pool from one of the core switch.

Is there any special parameter to set to mention a switch as DHCP master and standby?

Thank you

[devils_advocate]

I don't believe that is correct.

A DHCP Discover packet is a broadcast so will go to both Routers, not just the active one.

My understanding is that both routers could assign addresses so you could end up with duplicates.

The Routers DHCP services will have to rely on pings to avoid conflicts which is not 100% effective.

I believe the only solutions are to split the DHCP scopes between the routers, i.e 50% on each or use a Windows Server 2012 or upwards which is capable of dealing with redundant DHCP scopes.

HSRP is about providing the clients with a 'redundant' default gateway as such so the clients would have no concept of it until they have an IP address.

Thanks