×

Warning message

  • Cisco Support Forums is in Read Only mode while the site is being migrated.
  • Cisco Support Forums is in Read Only mode while the site is being migrated.

OSPF dr and bdr

Answered Question
Jul 29th, 2012
User Badges:

Hello, I have attached an image of a network I am working on.

I have enabled OSPF on all five routers.


I notice that only the link (circled in pink) between 'xyzCompany' and 'Admin' routers has elected a DR (xyzCompany) and a BDR (Admin).

This link between the two is classified as a "BROADCAST" link rather than a "POINT TO POINT" link.


I am still learning about OSPF. Can someone please tell me why it is only the link between these two routers that is configured with a DR and a BDR and the broadcast classification, while the other router links dont have a DR/BDR and are classified as point to point links ?


Thanks for any explanation

Attachment: 
Correct Answer by Richard Burts about 5 years 2 weeks ago

It is a bit difficult to be sure but it looks in the drawing like the other links are serial point to point links. Can you confirm that this is the case?


OSPF makes a distinction about network types. For networks like Ethernet where there potentially could be multiple neighbors on an interface OSPF uses the concept of DR and BDR. After election of DR and BDR then all OSPF routers on the segment form adjacency with the DR and with the BDR (but not with other neighbors on the segment).


On serial point to point links (PPP, HDLC, Frame Relay point to point subinterface, etc) OSPF knows that there can not be more than one neighbor on the segment. Therefore there is no need to the mechanism of DR and BDR and so the OSPF routers immediately form adjacency with each other.


HTH


Rick

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Correct Answer
Richard Burts Sun, 07/29/2012 - 20:54
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

It is a bit difficult to be sure but it looks in the drawing like the other links are serial point to point links. Can you confirm that this is the case?


OSPF makes a distinction about network types. For networks like Ethernet where there potentially could be multiple neighbors on an interface OSPF uses the concept of DR and BDR. After election of DR and BDR then all OSPF routers on the segment form adjacency with the DR and with the BDR (but not with other neighbors on the segment).


On serial point to point links (PPP, HDLC, Frame Relay point to point subinterface, etc) OSPF knows that there can not be more than one neighbor on the segment. Therefore there is no need to the mechanism of DR and BDR and so the OSPF routers immediately form adjacency with each other.


HTH


Rick

fran19422 Sun, 07/29/2012 - 21:04
User Badges:

thank you. That is a great help.

Yes, the other links as you mention are all serial point-to-point links.


Can I please ask which of the two routers on the ethernet connection eg.'xyzBldg' and 'Admin' would be the better one to make the DR ?

i.e. I would like to know why you would choose one over the other if I was manually setting the DR.


Thank you kindly.

Joseph W. Doherty Mon, 07/30/2012 - 02:58
User Badges:
  • Super Bronze, 10000 points or more

Disclaimer


The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.


Liability Disclaimer


In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.


Posting


Packet tracer?


If the link between the pair of 1841 is a p2p Ethernet, there's an OSPF command to have OSPF treat the link as p2p.  Normally, this would be the optimal option in this case.


Otherwise, as DR often does a little more "work", when given a choice, you chose a router for DR that has more (available) capacity.  Difficult to know exactly which this would be, but in this case, since Admin is an edge router, it might make the better DR for the one link.


PS:

The interface command to force ospf p2p (for your Ethernet p2p) is:

ip ospf network point-to-point

Richard Burts Mon, 07/30/2012 - 11:04
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Since it looks like both of these routers are the same model (and we assume similar capacity) then I would mostly agree with the suggestion from Joseph to use the router that may be more lightly loaded.


But I would say that in this situation I see very little point in manually setting the DR. Setting the DR does introduce some complexity into the config and I see very little benefit from doing this.


I would also note that setting the config so that a certain router becomes DR in the beginning it does not assure that this router will continue to be the DR. It is quite likely that after the routers have been running for some time that the other router will have become DR. Perhaps when the network first comes up that Admin will be DR. But in a few days (or a few weeks) perhaps Admin is rebooted during a maintenance window. When Admin reboots the other router takes over as DR. And when Admin comes back into service it will not automatically become the DR. So my advice is to keep it simeple and use the defaults for selection of DR.


HTH


Rick

Richard Burts Mon, 07/30/2012 - 11:17
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

I have had an additional thought and want to change/clarify my previous answer. Sine this environment has only two routers it makes absolutely no sense to try to set the DR and would make no difference if you do try.


With two routers one will become DR and one will become BDR, and they will both form adjacency with each other. And there is no difference in the amount of work that the DR does.


HTH


Rick

Actions

This Discussion

Related Content