“A protocol used by Layer3 devices (like Routers) to discover neighbors and form adjacencies with each other”
When a protocol process starts on a router, neighbors must be discovered and adjacencies must be established. Hello is a protocol which performs this duty.
Hello protocol performs several functions, including but not limited to:
- It discovers neighbors (such as OSPF or IS-IS routers)
- It performs three-way handshaking to ensure bidirectional communication between the neighbors.
- It communicates information necessary for establishing whether an adjacency can be formed with a neighboring router
- After an adjacency is formed, it serves as a keepalive mechanism to detect failed neighbors or adjacencies
In reality, the Hello Protocol is responsible for establishing and maintaining neighbor relationships. It also ensures that communication between neighbors is bidirectional. Hello packets are sent periodically out all router interfaces. Bidirectional communication is indicated when the router sees itself listed in the neighbor’s Hello Packet. On broadcast and NBMA networks, the Hello Protocol elects a Designated Router for the network.
The Hello Protocol works differently on broadcast networks, NBMA networks and Point-to-MultiPoint networks. On broadcast networks, each router advertises itself by periodically multicasting Hello Packets. This allows neighbors to be discovered dynamically. These Hello Packets contain the router’s view of the Designated Router’s identity, and the list of routers whose Hello Packets have been seen recently.
On NBMA networks some configuration information may be necessary for the operation of the Hello Protocol. Each router that may potentially become Designated Router has a list of all other.
Hello protocol uses an algorithm similar to Distance Vector algorithm. But Hello does not use hop count as a metric. Instead, it attempts to select the best route by assessing network delays and choosing the path with the shortest delay.
OSPF has 5x types of Packets … one of which is Hello. Below is OSPF Hello Packet format:
OSPF uses Multicast to send Hello packets for Broadcast and point-to-point network types. These packets are addressed to the all OSPF Routers Multicast group address 224.0.0.5.
Does hello protocol behaviour same on ospf and is-is?
In IS-IS, Hello interval is 10sec & Dead interval is 30sec by default. Interface’s hello timer can be adjusted in case of both the major IGP i.e. OSPF & IS-IS.
What will be Hello/Dead timers in network having link capacity of >T1 and <=T1?
Hello & Dead Intervals don’t depend on the link capacity. It depends on the port/link type e.g. Broadcast or p-p links.
The important point is that dead interval is exactly four times than Hello and its value can be set manually in most of the vendor equipment.
For example, For a 1.544Mbps link, if Hello takes 10 seconds then Dead will be 40 seconds. In another example, if Hello is 30 seconds then Dead will be 120 seconds.
Very nice Post … Really appreciated.
Please if you can add some light on BGP as well.
Yes, please have a look at the BGP section!
I was referring to BGP practically