DHCP Failover - initial Configuration

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Aug 8 17:24:04 UTC 2018

Philippe Maechler <plcmaechler at gmail.com> wrote:

> If we wanna go for failover, what are the right steps to start?
> 	• Configure failover on the primary node (in the local confg)
> 	• Choose which we wanna do failover and configure them
> 	• Restart the primary node and put it into partner down state
> 	• Configure the second server (failover and pools)
> 	• Start the second server
> 	• Put the primary server into partner-up? Mode
> Does this sound right?

Almost, the last step is automagic - when the second server comes up, it will communicate with the first, sync the leases, then after (AIUI) MCLT they will both go into normal operation.

> /30 networks
> We have about ~240 pools, ~50 pools only contain one single ip address. Does failover makes sense here?
> We can’t use hosts definitions because we only know the option-82. Some customers have more than one device connected but we can only serve the single ip address to one of them. If we use failover, can it happen, that server-1 hands out the ip address to device-1 and server two hand out the ip to device-2?

Failover won't work with such a pool - there's no free leases to balance between the servers. You could configure the same pool on both servers without failover - but then, as you suggest, the same address could be leased to two devices.

> Heavily used pools
> The bigger part is our /24 pools. These are all in a shared network config. I guess the failover part works pretty fine for the individual pools here. The shared network is sometimes at 95% usage. Can this lead to problems?

As long as there are free leases in a pool then it will work.

> Server restarts
> Currently we restart the service every 5minutes if something changed. When we go for failover, we should reload server one and if it synced to his partner, we can reload the server two. How does server two know, that the server one is up to date and everything is synced?

After a restart it will take time for the servers to resync. You'll need to adapt your management system to hold off on restarts. Hopefully someone more familiar with failover will be along soon with more details, but from things said on here, there are some cases where the servers can take a while before they get back to fully normal operation.

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