OSPF convergence during all priority router down

kthned · ‎03-24-2010

Hi

I have question related to OSPF Router Priority.What will happen to the election process when all the routers in the NBMA has zero proiority. In other words, will OSPF converge if DR and BDR goes down and the rest of the router in the system has priority set to zero.

Looking forward for guidance.

Thanks !

Regards,

Umair

Tharak Abraham · ‎03-24-2010

Syed,

Its true we have a DR in a normal NBMA design but keeping a BDR is very less likely unless we have a fully meshed NBMA network.

Fully meshed meaning the spokes have links/seperate DLCI's to other spokes as well.

You know that the DR needs to have connectivity to all other routers (think of Hub and spoke over here) and in NBMA design we would always want the Hub/Main router/Central office router to be the DR and not any branch office routers for the above obvious reason that this router would have seperate DLCI's running to all the individual spokes

Hence we dont even give a chance for the branch office routers to participate in the election by assigning a zero priority.

If the DR or central router goes down and the entire network is down

Since you are running OSPF on NBMA please consider the other modes like point to multipoint which is by far the most recommended method.

No DR election takes place but all the more in any partially designed topology with NBMA, when you lose physical connectivity to the main router, consider the network to be broken !

kthned · ‎03-24-2010

So thats mean if I dont have DR candidate (meaning all router has prio = 0) then potentially, no updates because of no DR in the NBMA. correct ?

what do you mean by point to multipoint, I didnot get the point.

Tharak Abraham · ‎03-24-2010

Yes, if you dont have any other backup candidate in your FR network then consider the network broken !

Now for eg:

If you fully mesh your FR sites, with all routers having seperate DLCI's/virtual circuits to all others then we could exactly emulate the broadcast medium and have DR and BDR and all that jazz !

Point to multipoint is a mode for OSPF to run over NBMA and it doesnt require the DR to pass on updates or be responsible for anything.

But we do need a Central router which has connectivity to all others in this case too.

I would recommend reading the below blog by Mr.Arden Packeer and verify these modes in a lab for fully understanding !

http://ardenpackeer.com/routing-protocols/tutorial-ospf-network-types-and-frame-relay-part-2/

ravishankar.ramakrishnan · ‎03-24-2010

OSPF configuration depends upon the scenario and connectivity of the ospf participating routers.

To look at there are various OSPF network types:
Broadcast Multiaccess
Point to Point
Non Broadcast Multiaccess

NBMA type of network also runs in five modes:

RFC Standard:
1. NBMA : DR/BDR election and Static Neighbor
2. Point to Multipoint : No DR/BDR election and Auto Neighbor Discovery

Cisco Standard:

3. Point-to-Point : No DR/BDR election, Sub interface, Auto Neigbhor Discovery

4. Broadcast: DR/BDR election, Full mesh and Auto-discovery

5. Point-to-Multipoint Non Broadcast: No DR/BDR, Static Neighbor configuration

So, here the point is, based on the OSPF network we can design the configuration.

In NBMA mode, OSPF needs the DR/BDR election and having the router priority is essential for election of DR/BDR. Priority ZERO means ROuter will not take part in DR/BDR election. In this case if there is no DR/BDR election then there is no neighbor relationship taking place and so network is down.

Priority ZERO is only used when there is a branch router which is not having full connectivity to all other router and they should not take place in the DR/BDR election.

However in case of Point to Point and Point-to-Multipoint mode the DR/BDR election is not needed. In that case even if the priority is ZERO, there will be neighbor relationship established and network is up.