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OSPF DR and BDR

Posted on 2014-09-18
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Last Modified: 2014-10-01
I would like to know in which topology DR/BDR will be formed.
I know that it automatically forms in a Broadcast segment, but I wonder if it means full mesh ?

example:
R1 is connected to R2 and R2 is connect to R3 and R3 is connected back to R1, This is Broadcast segment and the DR/BDR election will occur..

What if :
R1 is connected to R2 and R2 is connect to R3 but  R3 is not connected  to R1. I believe this is not Broadcast and not full mesh, as such no DR/BDR election...

What if:
R1 is connected to R2 and R2 is connect to R3 and R3 is connected back to R1,and there is another R4 connected only to R1... would R4 participate in DR/BDR elections ?

Thanks
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Question by:jskfan
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by:Akinsd
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DR and BDR will always be elected in a LAN environment whether full or partial mesh.

By default, every OSPF device is a DR until it sees another device on the network that supersedes it.

There is a way to even configure devices in a WAN environment to for DRs and BDRs eg Frame relay point to multipoint because the connection simulates a LAN environment.

Long story short, if it is a LAN environment, DRs and BDRs will be formed as long as there are 2 or more routers running OSPF
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by:Otto_N
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Whether OSPF attempts to elect a DR/BDR depends on the interface type it runs on - Only if it is classified as a broadcast network, will it attempt to elect DR/BDR.  On Cisco Routers, Ethernet and VLAN interfaces are by default broadcast (where DR/BDR election occur), and serial, ATM, POS and others are classified as point-to-point (no DR/BDR election takes place).  These default setting can be overridden in some instances, if you choose to.

Also, keep in mind that DR/BDR election also only occur per segment.  For example, in your scenarios, if the links between the routers are not in the same segment (i.e. same broadcast domain), R1 & R2 will elect a DR/BDR between them for the R1-R2 link, R2 & R3 will elect a DR/BDR between them for the R2-R3 link, and so on, if the links are classified as broadcast.  If they're in the same segment, all the routers that form part of that segment will participate in a single DR/BDR election process.

It is possible to configure multi-access networks on something like Frame-relay, and if a full-mesh is not provided, the default OSPF DR/BDR election process won't work reliably - If you do not have a full mesh, OSPF adjacencies may form, but OSPF may not converge.  It is then better to configure it as a bunch of point-to-point links.
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by:jskfan
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<<<Also, keep in mind that DR/BDR election also only occur per segment>>>
the only way I see you can connect 3 or more routers into the same segment is if you use a Switch.

if you  connect only R1 to R2, that will be point to point ,regardless if it is serial or Ethernet interface. so there will be no DR/BDR . correct ?

if you connect R1 to R2 and R2 to R3,  they got to be in different segments, and there will be no DR/BDR. correct ?

I also want to confirm that DR/BDR not only requires routers to be in the same network segment, but also in the same OSPF area . Correct ?

Thanks
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by:Don Johnston
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the only way I see you can connect 3 or more routers into the same segment is if you use a Switch.
Or frame-relay or MPLS.

if you  connect only R1 to R2, that will be point to point ,regardless if it is serial or Ethernet interface. so there will be no DR/BDR . correct ?
No. If it's an OSPF Broadcast network type, there will be a DR & BDR even with just two routers.

if you connect R1 to R2 and R2 to R3,  they got to be in different segments, and there will be no DR/BDR. correct ?
No, they can all be on the same segment/network/VLAN/broadcast domain.

I also want to confirm that DR/BDR not only requires routers to be in the same network segment, but also in the same OSPF area . Correct ?
Correct.
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by:jskfan
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<<<if you connect R1 to R2 and R2 to R3,  they got to be in different segments, and there will be no DR/BDR.>>>

I need to try it....because in this case R2 will have 2 different interfaces in the same subnet... I do not think it will let you do that...
Can R2 have one interface with 192.168.1.10 and another interface with 192.168.1.20 ??
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by:Don Johnston
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Is there a diagram somewhere that I missed?
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by:Otto_N
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@Don - No, there's no diagram, just the problem statement by jskfan.  I also assume that this is more a theoretical discussion than a solution to a practical problem, so I suspect we'll need multiple diagrams to cover all the options...

@jskfan
What Don refers to, is that the way you connect the routers will determine how the DR/BDR election will happen - if R1, R2 and R3 are connected to the same broadcast domain (a.k.a "a segment"), they will elect a single DR/BDR between them.  This single segment does not need to be an Ethernet switch - In the good old days Frame Relay could provide this feature, but nowadays there are many technologies that can do the same thing.  But I digress...

Regarding the sharing of a subnet between interfaces: From a router's point of view, a single subnet implies a single broadcast domain, i.e. single segment.  While a router will not give you an error if you configure two interfaces in the same subnet (it cannot tell if you're planning to connect it to the same Ethernet switch, for example) it will not route properly if these two interfaces are not connected to the same segment/broadcast domain.

Rule of thumb: One IP subnet => One broadcast domain.

Now, lets get back to your question:
Example: R1 is connected to R2 and R2 is connect to R3 and R3 is connected back to R1,
If I read this, it implies that the R1-R2 link is a different segment than R2-R3 and R3-R1.  Then, there will be either a DR/BDR per segment (if it is a "broadcast" medium used for the connection, like an Ethernet cable), or there will be no DR/BDR elected on a segment (if it is a "point-to-point" medium used for the connection, like a serial interface).  There will not be a single DR/BDR election between all three routers, as you presumed.

In my opinion, your questions then become self-explanotary:
What if R1 is connected to R2 and R2 is connect to R3 but  R3 is not connected  to R1.
Then R1-R2 will elect a DR/BDR (or not, depending if the link is "broadcast" or "point-to-point"), same with R2-R3.
What if R1 is connected to R2 and R2 is connect to R3 and R3 is connected back to R1,and there is another R4 connected only to R1... Would R4 participate in DR/BDR elections
Again, this depends on the actual link between R1&R4.  If it is a different segment/broadcast domain from the R1-R2 and R1-R3 links (as I assume), then R1 & R4 will elect a DR/BDR (or not, depending if the link is "broadcast" or "point-to-point").

I hope this helps.
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by:jskfan
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Ok...
I did not know that DR/BDR depends also on medium (Ethernet vs serial)
To me point to point means there is no switch or else between the routers...
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by:Don Johnston
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Well, there is no "official" definition of "point-to-point" so it could mean whatever you want it to.  ;-)

Another way of defining it would be a medium where there can only be two hosts. For example, an ethernet link between two hosts with a /30 mask could be considered a point-to-point link.

But most people would just think of point-to-point leased lines (T1, T3, etc.).
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An Ethernet cable between two hosts with a /30 mask can be considered a point-to-point link, but the OSPF process on a router would still classify it as a BROADCAST network type (thereby initiating DR/BDR election), since the media (Ethernet) is inherently multi-access.  If you want to check what OSPF classify the interfaces as, you can do a "show ip ospf interface" command (on Cisco IOS devices):
Router#show ip ospf interface
Serial0/0/0:1 is up, line protocol is up
  Internet Address 10.1.1.18/30, Area 0
  Process ID 850, Router ID 10.1.0.4, Network Type POINT_TO_POINT, Cost: 48
  Transmit Delay is 1 sec, State POINT_TO_POINT
...
Vlan851 is up, line protocol is up
  Internet Address 10.1.3.17/30, Area 0
  Process ID 850, Router ID 10.1.0.4, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1

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I also know that you can change the default behaviour per interface - If you take the previous example of an Ethernet cable connecting two routers, you can configure the two routers to treat the interface as a point-to-point link, to eliminate the DR/BDR election process.  The Cisco IOS command to accomplish this is "Router(config-if)# ip ospf network [ broadcast | point-to-point | ... ]"
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by:jskfan
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Thank you Guys!
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