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Saturday, July 21, 2012

OSPF network types - Part 1

This time i will examine the options for OSPF network types.
Let's take the following scenario:

Side note – as part of my preparation to the CCIE R&S lab exam I keep practice and configuring manually a cisco router as frame-relay switch instead of using the built-in one.

Here we have a hub and spoke topology where R1 is the hub and R2 and R3 are the spokes. R1 is configured with interface s0/0.123 multipoint were R2 and R3 are configured with sub-interface point-to-point.

After finishing configure the interfaces (encapsulation, fram-relay, IP's) and OSPF process i didn't manage to see adjcencies...!

here is a snippet from the configurations:

interface Serial0/0.123 multipoint
 ip address
 ip ospf network point-to-multipoint
 snmp trap link-status
 frame-relay map ip 102 broadcast
 frame-relay map ip 103 broadcast
 frame-relay interface-dlci 102
 frame-relay interface-dlci 103
 no frame-relay inverse-arp

interface Serial0/0.12 point-to-point
 ip address
 ip ospf network point-to-point
 frame-relay interface-dlci 201  


interface Serial0/0.13 point-to-point
 ip address
 ip ospf network point-to-point
 snmp trap link-status
 frame-relay interface-dlci 301  

So first let's examine this situation more deeply, we have a hub router which connected to two spoke routers, so the hub is multipoint while the two spokes are point-to-point.

Now OSPF network type Point-to-Multipoint uses the following:
LSA flooding - Multicast
DR/BDR Election - No
Timers - 30/120
Neighbor statement - No
Modify next-hop - yes

while point-to-point network type uses:
LSA flooding - Multicast
DR/BDR Election - No
Timers -10/40
Neighbor statement - No
Modify next-hop -No

3 basic things we should check if adjacency between OSPF neighbors doesn't come up:
1. LSA flooding method (unicast or multicast)
2. MTU
3. Timers

As you may recall OSPF adjacencies never comes up if the timers are not the same, in my example the multipoint uses 30/120 while PtP uses 10/40.

In order to fix the problem configure ip ospf hello-timer, in the interface level, on the hub router to match the spokes timers.


  1. Excellent blog, I loved it so much that I came back here to give feedback. Thanks though.

  2. there is so mmucch to learn about the OSPF network in this posts. thanks for sharing it. it's very helpful.

  3. Thanks for the detailed article on this topic. I would like to see more such awesome articles from you.

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