Hello everybody, today we will talk about OSPF Hello packets. Wait! Before you go away from here dismissing this as the most elementary thing in the networking world, give me a minute or two to get you understand what specifically we will deal with in this post.

Are OSPF Hellos Multicast packets? The answer to that question should ideally be a resounding ‘Yes’ but we do have a catch. The catch is called ‘Immediate hellos’ which are not Multicast packets but are plain Unicast packets. So, what exactly are these hellos? Basically it is a way of forming the OSPF adjacency faster than it is supposed to. How does it help? Simple! It helps in faster convergence of networks. Generally, if we look at the OSPF design, by default, it will send out a Hello packet every 10 seconds to it’s multicast neighbors using the Multicast IP address. In this case, or if we can say with this kind of implementation, the Hub device [let’s say the one who sent the first hello on the network] will send out an Immediate Hello upon receiving the Multicast hello from the Spoke. This, essentially, saves our 10 secs! The Network Sherpa discusses a bit more on this topic.

Remember, this is dependent on implementation, luckily the Cisco router I am working on right now supports this kind of an implementation, this might not be applicable to your router. Who knows? Go, enable debugs and check it out for yourself!

If you look at the logs below:

 

.Aug  1 06:26:26.706: OSPF-1 HELLO Gi0/0/1: Send hello to 224.0.0.5 area 0 from 10.1.1.1

.Aug  1 06:26:26.706: OSPF-1 PAK  : Gi0/0/1: OUT: 10.1.1.1->224.0.0.5: ver:2 type:1 len:44 rid:1.1.1.1 area:0.0.0.0 chksum:EA9C auth:0 –> That is the first Multicast hello packet going out.

.Aug  1 06:26:28.333: OSPF-1 PAK  : Gi0/0/1:  IN: 10.1.1.2->224.0.0.5: ver:2 type:1 len:44 rid:3.3.3.3 area:0.0.0.0 chksum:E698 auth:0 –> That is the first Multicast hello packet coming in.

.Aug  1 06:26:28.333: OSPF-1 HELLO Gi0/0/1: Rcv hello from 3.3.3.3 area 0 10.1.1.2

.Aug  1 06:26:28.333: OSPF-1 HELLO Gi0/0/1: Send immediate hello to nbr 3.3.3.3, src address 10.1.1.2

.Aug  1 06:26:28.333: OSPF-1 HELLO Gi0/0/1: Send hello to 10.1.1.2 area 0 from 10.1.1.1

.Aug  1 06:26:28.333: OSPF-1 PAK  : Gi0/0/1: OUT: 10.1.1.1->10.1.1.2: ver:2 type:1 len:48 rid:1.1.1.1 area:0.0.0.0 chksum:E492 auth:0 –> This is our man! The Immediate Unicast Hello going out right after the Multicast hello came in. This is seen on both routers, Hub and Spoke. Check for the time-stamps, not even one sec was wasted. That was some fast packet processing and creation!

.Aug  1 06:26:28.338: OSPF-1 PAK  : Gi0/0/1:  IN: 10.1.1.2->10.1.1.1: ver:2 type:1 len:48 rid:3.3.3.3 area:0.0.0.0 chksum:E492 auth:0 –> The Immediate Hello coming in from the Spoke. Beyond this, everything is Multicast again.

.Aug  1 06:26:45.892: OSPF-1 HELLO Gi0/0/1: Send hello to 224.0.0.5 area 0 from 10.1.1.1

.Aug  1 06:26:45.892: OSPF-1 PAK  : Gi0/0/1: OUT: 10.1.1.1->224.0.0.5: ver:2 type:1 len:48 rid:1.1.1.1 area:0.0.0.0 chksum:E492 auth:0

.Aug  1 06:26:47.737: OSPF-1 PAK  : Gi0/0/1:  IN: 10.1.1.2->224.0.0.5: ver:2 type:1 len:48 rid:3.3.3.3 area:0.0.0.0 chksum:E492 auth:0

 

I hope this was an interesting piece of information to store in your head. Please let me know if you have any queries on this.

Write to me, or simply post comments below on what you would want to know, discuss here! I’ll be back with more, another day.

Take care, Adios!

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