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OSPF Hello Timers

OSPF hello packets serve a dual purpose: establishing neighborships and acting as keep-alives to detect when adjacent routers go offline. Understanding hello and dead interval timers is essential for controlling how quickly OSPF responds to topology changes.

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About this video

OSPF hello packets serve two distinct functions: forming initial neighborships and maintaining them over time through a keep-alive mechanism. Routers continuously exchange hello packets at a set interval, and as long as those packets keep arriving, each router knows its neighbor is still operational. When a router stops receiving hellos from a neighbor, it waits a defined period before declaring that neighbor unreachable, removing it from the neighbor table, and recalculating the best available path to affected networks. Two timers govern this process. The hello interval controls how frequently hello packets are sent — defaulting to 10 seconds on point-to-point and broadcast links, and 30 seconds on non-broadcast multi-access connections. The dead interval determines how long a router will wait without receiving a hello before considering the neighbor down, and it defaults to four times the hello interval: 40 seconds for broadcast networks and 120 seconds for non-broadcast multi-access environments. For a neighbor adjacency to form successfully, both routers must be configured with identical hello and dead interval values on the connecting interfaces. Although the defaults cover most deployment scenarios, these timers can be customized on a per-interface basis when specific conditions warrant it — such as conserving bandwidth on a lower-capacity link or reducing convergence time on a critical, high-stability connection. Any manual changes must be applied consistently on both ends of the link to maintain a valid adjacency.

What you'll learn

What's covered

OSPF Hello Timers

Aligned to

Cisco CCNA
3.4 Configure and verify single area OSPFv2

Key terms

Open Shortest Path First
OSPF
A link-state interior gateway routing protocol that maintains a complete map of the network topology and uses Dijkstra's Shortest Path First algorithm to calculate optimal, loop-free routes. OSPF divides networks into hierarchical areas to improve scalability and supports VLSM and rapid convergence.
Hello Packet
An OSPF control message sent periodically on active interfaces to discover neighbors, verify reachability, and maintain adjacencies.
Keep-Alive
A mechanism in which periodic messages are sent between devices to confirm that a neighbor or connection remains active and operational.
Hello Interval
The configurable timer that defines how frequently an OSPF router sends hello packets out of an interface to maintain neighbor adjacencies.
Dead Interval
The configurable timer that defines how long an OSPF router waits without receiving a hello packet before declaring a neighbor down; defaults to four times the hello interval.
Non-Broadcast Multi-Access
NBMA
An OSPF network type that supports multiple routers but does not natively support broadcast, resulting in a default hello interval of 30 seconds and a dead interval of 120 seconds.

Transcript

Hello Packets as Keepalives

We've talked about how OSPF uses hello packets to create neighborships with other routers on the same link. However, these OSPF hello packets serve another really important purpose, and that is something called a keepalive. It allows the routers to know if that neighboring router is still up and running.

If there's two routers that have created adjacencies and are talking back and forth, and one minute a router goes down, that other router needs to know that it's down and needs to notify the rest of the network that that link is no longer viable. So what these routers will do is send hellos every 10 seconds, every 30 seconds, or whatever you set it to, to notify "hey, I'm still up and running." We call these keepalives. It keeps the link alive, it keeps the neighborship alive.

So let's go over in our network here. Router 2 has a neighborship with router 3. They've exchanged hello packets and are aware of each other. Now, if router 3 were to go down, router 2 may still continue to send traffic in that direction; it may not know that router 3 is down. So the hello packets will continue to send just to make sure that that neighborship stays alive. It will send every 10 seconds, every 30 seconds over to router 2 and just let it know, hey, I'm still up. And if a certain period goes by and router 2 does not receive a hello packet from router 3, it's going to assume that router 3 is down, and then it's going to take it out of its neighbor table and readjust to try to send to this network if there's an alternate route to that network.

The Hello Interval

Hello timers determine how chatty your interface is going to be. One of those timers is the hello interval. The hello interval is going to be how often the hello packets are going to be exchanged. So for instance, if we have a hello timer of 10 seconds, then that means every 10 seconds router 3 is going to send a message to router 2, and vice versa: every 10 seconds router 2 is going to send to router 3. So that's how often they're going to exchange those hello packets.

10 seconds is the standard for a point-to-point or a broadcast network. We have a point-to-point and HDLC set up on these two links, so that would qualify as that, so these by default would be 10 seconds. Every 10 seconds it's going to be sending back and forth.

Alternatively, we could have a non-broadcast multi-access or a point-to-multipoint network. These are ethernet connections over here and here, and so those ethernet connections are non-broadcast multi-access, so the default for that is 30 seconds. So it's going to vary depending on the interface that we have.

The Dead Interval

So that's the hello interval. The dead interval is going to be: if I don't hear from you in this amount of time, then I'm going to assume that you are down. The dead interval for these connections is four times whatever the hello interval is going to be, so 40 seconds for the broadcast networks, and it's going to be 120 seconds for these non-broadcast multi-access networks. That is the dead interval. If I haven't heard from you in 40 seconds or 120 seconds, I'm going to assume that you're down and then figure out a different way to get to those networks. I'm going to recalculate.

Timers Must Match

When we're setting up these links and establishing these neighbor adjacencies, we do need these hello and dead intervals to be the same. Within that hello packet that we saw that's sent back and forth, these details go into that, and to create this adjacency they do need to be the same on both of the interfaces.

Changing the Defaults

Now, there are times that we may want to change the default for the hello interval and the dead interval, so we can actually configure this and change the default. If we were to do this, we want to once again do it on each of the sides, but this is a per-interface setting. So this link could be set to 10 seconds while this link maybe is set for 20 seconds. Maybe the bandwidth is not as large and so we want to conserve some bandwidth, or perhaps this link right here is a very stable link and we're not concerned with it going down as much. There might be some reasons why we'd want to change this.

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