OSPF over FrameRelay..No BDR/DR on multipoint- this normal?

Lessons Discussion
1

I knew on point to point there was no DR/BDR selection. I knew that DR/BDR selection ran over 224.0.0.6 multicast, and the other 224.0.0.5

I have a hub and spoke topology with two PVC. On this I ran OSPF and used “ip ospf network point-to-multipoint” on the interface so that it would setup its neighbor stuff.

when setting up my frame mapping I used the broadcast on the end of my mapping.

everything works well and seems fine its just that there is no DR/BDR.

This gives me two questions.

  1. I thought the broadcast on the end of the (frame-relay map ip x.x.x.x DLCI “broadcast”) meant that those could send broadcast. Obviously not as I had to use the special OSPF cmd.

  2. I thought that since this was not point to point it would have DR/BDR

Can someone give some information here to help me understand this.

from my lab I understand that OSPF does not give you a DR/BDR period when ran over frame relay (not sure if I should accept that as fact or not as I have problems trusting my own findings a lot of times)

Also the broadcast has zero affect on routing protocols or if it does have an effect I don’t understand how it works.

2

Hello Brian

When you implement the ip ospf network point-to-multipoint command, essentially you are telling the router NOT to elect a DR/BDR.

If you want DR/BDR to be elected, then you must change the network type to ip ospf network broadcast. With this command on the interface, you must also include the frame relay map command as broadcast as well (as you said you have already done so).

To review, for OSPF with frame relay, you have the following different types of netowrks:

Point to Point configuration
This is where you have one interface connecting with another interface on one PVC where the link between the two is contained in one subnet. No DR/BDR is elected. In this case, as an example, you have the interface configured as follows:

interface Serial0.1 point-to-point
      ip address 1.1.1.2 255.255.255.0
      frame-relay interface-dlci 16

Multipoint configuration with broadcast network
This is where the network is viewed as a broadcast multi access network where DR and BDR are elected. An example configuration is the following. Notice the ip ospf network broadcastcommand:

interface Serial0.2 multipoint
      ip address 1.1.1.2 255.255.255.0
      ip ospf network broadcast
      frame-relay map ip 1.1.1.2 16 broadcast

Multipoint Configuration with Non-Broadcast Network
Here the network is non broadcast, so neighbours cannot be discovered automatically. With this configuration, neighbours must be manually configured. Once neighbours are created, DR and BDR are elected because of the multi access nature of the topology. In this configuration it is a good idea to An example configuration is the following:

interface Serial0.2 multipoint
      ip address 1.1.1.2 255.255.255.0
      frame-relay map ip 1.1.1.1 16 broadcast

Multipoint Configuration with Point-to-Multipoint Network
Finally, this configuration essentially allows the network to function as a collection of point to point links. Neighbours are discovered automatically, but DR and BDR elections DO NOT TAKE PLACE. An example configuration is the following:

 interface Serial0.2 multipoint
      ip address 1.1.1.2 255.255.255.0
      ip ospf network point-to-multipoint
      frame-relay map ip 1.1.1.1 16 broadcast

For more detailed information concerning OSPF and frame relay, take a look at http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13693-22.html#configs. It’s old but it is still valid.

I hope this has been helpful!

Laz

3

That helps thank you for posting.