VRRP and DHCP Interoperability: A Recipe for Connectivity Success

vrrp and dhcp

DHCP (Dynamic Host Configuration Protocol) really is one of those network services that makes a system administrator’s life easier on a day-to-day basis. However, it’s sometimes this very ease that can turn around and bite us in the backside if you don’t know how everything plays together.

Like a chef, engineers and administrators must be familiar with all the ingredients in their “pantry” if they want to produce the best final result.

So let’s start by considering how VRRP (Virtual Router Redundancy Protocol) and DHCP work together and the heart of the issue between these two protocols.

DHCP provides us the means to deliver IPs dynamically to network devices. VRRP, on the other hand, allows us to have two routers that share the same IP address as our default gateway so we can avoid a single point of failure on hardware or circuitry for a network exit point.

When building out small or large networks, what frequently causes grief is when we make the VRRP networking devices also responsible for handing out IP addresses (i.e. a DHCP server). Where the main issue lies is when these two devices are tied together, thus taking shared responsibility for handling the default gateway of a LAN subnet (VRRP) and preventing one another from synchronization when they are handing out IP addresses of the same two devices (think routers).

So how does this problem play out in real life?

As your network connection fails and the router responsible for the default gateway address (VRRP master) starts handing out IP addresses, it has no means of checking with the failed router to see what IP addresses it has already handed out. Hence, you have two devices handing out IP addresses for the same LAN subnet, with no means of communicating between the two.

That’s the bad news.

The good news is you can fix this issue to some degree by running an IP Helper address which points all DHCP requests back to a common DHCP database. Keep in mind, though, that this may cause other issues or may not meet your design criteria.

What we ultimately want is for the master VRRP device that owns the default gateway IP to update the VRRP slave’s DHCP database when it hands out IP address. That way if the VRRP slave is to become master, it will be aware of what IP addresses have already been handed out.

It’s a simple concept really, but has likely gotten lost in all the hype of SDN and IPV6.

So when it comes to DHCP and VRRP, tread lightly and understand the levels of interoperability they have. Remember: You don’t get any configuration red flags to inform you that something isn’t 100%. By understanding the different ingredients in your IT kitchen and how they work together, you can create a recipe for connectivity success.

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