OSPF Fast Hellos

The last day my colleague Abdullah Medhat posted a very nice topic about BFD technology “BFD – Bidirectional Forwarding Detection” that can detect a link failure in means of millisecond as mentioned but should be migrated with a routing protocol that support this technology, so that today I came to introduce an OSPF built-in technology that can do the same function – detect the failure in means of ms – of BFD without bringing up an extended adjacency or neighborships but depends on the OSPF itself which is OSPF Fast Hello packets.

Before we talk about it we should firstly know about the regular hellos and dead intervals in OSPF, and I know most of us do not like to check neither RFCs nor Cisco online documents so that we will start with a fast – short – recap.

What are the Hello packets and the dead interval?

They are the packets that an OSPF process sends to it’s neighbor to bring up the neighborship or maintain the connectivity with the already brought up neighbors. The hello packets are sent at a configurable interval in seconds. Here the question that comes up “what if a hello packet is lost before being received by the neighbor?” so that the neighbor will wait for the Dead Interval before turning down the neighborship and it is also configurable in seconds and the default is four times the value of the hello interval. and both hello and dead intervals must be the same across the network.

Now, after it became clear – I assume – how the hello packets and dead interval work together in mean of seconds to bring up the nighborship or maintain it, we might ask an intelligent question:  is there anything with OSPF that can detect the link failure in means of milliseconds like BFD? the answer is YES !! OSPF Fast hello packets.

SO, What are the OSPF Fast Hello packets?

OSPF fast hello packets refer to hello packets being sent at intervals of less than 1 second.

OSPF fast hello packets are achieved by using the ip ospf dead-interval command. The dead interval is set to 1 second, and the hello-multiplier value is set to the number of hello packets you want send during that 1 second, thus providing subsecond or “fast” hello packets.

When fast hello packets are configured on the interface, the hello interval advertised in the hello packets that are sent out this interface is set to 0. The hello interval in the hello packets received over this interface is ignored.

The dead interval must be consistent on a segment, whether it is set to 1 second (for fast hello packets) or set to any other value. The hello multiplier need not be the same for the entire segment as long as at least one hello packet is sent within the dead interval.

The below figures shows how to configure and validate the OSPF fast hello packets:















I hope you found this topic informative and useful…