cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Announcements
Join Customer Connection to register!
340
Views
15
Helpful
6
Replies
Ratheesh mv
Beginner

OSPF LSU

LSU.PNG

Hi all ,

 

When I captured OSPF packets I have noticed that there are unicast LSU and multicast LSU from the same device. What is the difference between each other. I have gone through many documents but there is no explanation about it .  

1 ACCEPTED SOLUTION

Accepted Solutions

Hello @Ratheesh mv ,

an adjacency is always built between a pair of OSPF speaking devices.

>> Are DBD and LSR always unicast packets or is there any exception cases ?

I think they are always unicast as the first describe the database  (LSA header, seq#, age time) not LSA body during Exchange state.

The second LSR lists a series of LSAs that the sender has noted as missing or older in its own DB and the packet is sent to stimulate a complete answer in form of LSU and here the LSA body contents is present

At the end of the process the two routers have their DB in sync and they reach the FULL state.

To be noted in a LAN segment with 4 or more routers non DR routers do not reach the FULL state between them they stop at Two ways. FULL state is reached with DR and BDR only. They are called neighbors they do not build an adjacency.

As this process requires a lot of resources DR and BDR had been introduced as a way to minimize the number of FULL adjacencies that need to be built and mantained over time.

15-20 years ago was quite common to have 10-20 OSPF routers or even more sharing the same segment interconnected via L2 switch.

 

As noted by @MHM Cisco World the OSPF network type does not from the number of neighbors but from the command

ip ospf network <type>

 

given in interface configuration mode.

 

Hope to help

Giuseppe

 

View solution in original post

6 REPLIES 6
MHM Cisco World
Collaborator

this is DR?
DR will unicast with each router to build one OSPF Database and then use multicast to flood it to all other router, this reduce the exchange between router in broadcast network type.

It is not a broadcast network .it is a point to point network type

Hello @Ratheesh mv ,

if the OSPF network type is point to point one possible explanation can be the following:

LSU sent as unicast are answers to LS Requests from other OSPF device

LSU sent as multicast are "unsolicited" and describe changes happened somewhere in the area and carry a new LSA or a new version of an LSA.

 

Hope to help

Giuseppe

 

Are DBD and LSR always unicast packets or is there any exception cases ?

Hello @Ratheesh mv ,

an adjacency is always built between a pair of OSPF speaking devices.

>> Are DBD and LSR always unicast packets or is there any exception cases ?

I think they are always unicast as the first describe the database  (LSA header, seq#, age time) not LSA body during Exchange state.

The second LSR lists a series of LSAs that the sender has noted as missing or older in its own DB and the packet is sent to stimulate a complete answer in form of LSU and here the LSA body contents is present

At the end of the process the two routers have their DB in sync and they reach the FULL state.

To be noted in a LAN segment with 4 or more routers non DR routers do not reach the FULL state between them they stop at Two ways. FULL state is reached with DR and BDR only. They are called neighbors they do not build an adjacency.

As this process requires a lot of resources DR and BDR had been introduced as a way to minimize the number of FULL adjacencies that need to be built and mantained over time.

15-20 years ago was quite common to have 10-20 OSPF routers or even more sharing the same segment interconnected via L2 switch.

 

As noted by @MHM Cisco World the OSPF network type does not from the number of neighbors but from the command

ip ospf network <type>

 

given in interface configuration mode.

 

Hope to help

Giuseppe

 

View solution in original post

Even if you connect back to back two router if you use Ethernet media this will behave as broadcast.

244.0.0.5 for multicast for broadcast network.

to make this Ethernet back to back point to point you must config it otherwise this by default broadcast