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OSPF-- Do the DR & BDR need to be adjacent?

MicJameson
Beginner
Beginner

Hi.

OSPF...

1. Do the DR & BDR need to be adjacent, or can they exist many hops away from each-other?

2. In a vinella architecture, do ALL OSPF routers in the same area establish at least a 2-way relationship with all other routers many hops away, or only with adjacent routers?

Thank you!

1 Accepted Solution

Accepted Solutions

"When I said "many hops", I meant many jumps from switch to switch."

I suspected that's what you meant, although, in networking, when we mention "hops", we often are referring to L3 hops, jumping/transiting across/between networks.

"Let's say there exists an ethernet network in a ring with 8 routers and evenly spaced out among 80 switches. 3 vlans exist in this network, and you place the DR and BDR maximally far from eachother. Would this work just fine?"

To my knowledge (discounting number of VLANs - more on that next), yes, I would expect it to work just fine (I'm assuming distance latency isn't an issue, either).  (Note in my prior post I even mentioned L2 OSPF neighbors on other sides of the world.)

"Am i correct in understanding that the OSPF instance is not affected by the amount of vlans, because the routers communicate through multicast, and all is needed is to input..."

Yes and no.  OSPF, overall, on any one device, is impacted by the amount of work required to maintain it.  A DR does more "work", so possibly, more VLANs, more DR instances, more "work", can impair OSPF.  For this reason, you can prioritize, DR election selection, even fully excluding a DR election (per device/network), altogether.

Sort of like if you have multiple routers acting as HSRP gateway on multiple networks, you can "spread the load" by determining which router is the primary HSRP gateway per VLAN.  With more routers and/or networks, you have more choices how to "spread the load".  Basically, ditto for DRs.

Also, like with HSRP, you might determine what router is acting as DR for shared networks also based on the overall "power/performance" capacity of the router.

View solution in original post

12 Replies 12

paul driver
VIP Expert VIP Expert
VIP Expert

Hello
1-No but Dr/BDR elections are usually used when you have multiple ospf devices that share the same address range via a single llayer 2 broadcast link to form a full mesh relationship so in away they are all connected to each other.

if you only have two ospf devices within an address range then they don’t require any DR/BDR they can become point-2-point peerings 

2- Default all ospf enabled Ethernet links are set to a broadcast network type thus participate in DR/BDR election once they have become full adjacenct with a DR/BDR any othe adjacency will establish a 2way state within that broadcast domain


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

It just occurred to me...

It seems to me that different VLANS (layer 2 separation) would need to run different instances of OSPF. Is this true?

Hello
If you require ospf on the SVIs then each SVI will need to be ospf enabled, with just a sinlge instacne of ospf.

example:


int vlan x
ip ospf 1 area 0

int vlan y
ip ospf 1 area 0

Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

Can you more elaborate?
the DR/BRD is important if you run Hub and Spoke

Perhaps there is something in the original post that I am not understanding correctly. But when I consider this part of the post "can they exist many hops away from each-other?" I assume that many hops describes connections through multiple subnets. And if the devices are in different subnets then they can not be DR or BDR with each other. Sometimes people believe that there is a single DR and a single BDR per OSPF network. But this is not correct. There is a DR (and perhaps a BDR) per subnet/network (in the same layer 2 broadcast domain).

HTH

Rick

As Rick correctly notes, DR and BDR (and the other routers using the DR), need to be on the same (logically) shared L2 media.  Physically, the routers could be widely separated, even if on other sides of the world, but logically, the need to appear and be "visible" as neighbors on the same L2 media.

As Rick also correctly notes, a DR or BDR is per network, and one router, for different networks it connects to could be the active DR for none, one or many, active BDR for none, one or many, electable DR/BDR for none, or many, non-electable DR/BDR for none, one or many.

Thank you all for your thoughtful replies. I do appreciate it.

When I said "many hops", I meant many jumps from switch to switch. May you please confirm that my understanding was wrong on this specific point?

Let me make a clarifying example--

Let's say there exists an ethernet network in a ring with 8 routers and evenly spaced out among 80 switches. 3 vlans exist in this network, and you place the DR and BDR maximally far from eachother. Would this work just fine?

Am i correct in understanding that the OSPF instance is not affected by the amount of vlans, because the routers communicate through multicast, and all is needed is to input...

"int vlan x
ip ospf 1 area 0
int vlan y
ip ospf 1 area 0"     ?

Thank you!

"When I said "many hops", I meant many jumps from switch to switch."

I suspected that's what you meant, although, in networking, when we mention "hops", we often are referring to L3 hops, jumping/transiting across/between networks.

"Let's say there exists an ethernet network in a ring with 8 routers and evenly spaced out among 80 switches. 3 vlans exist in this network, and you place the DR and BDR maximally far from eachother. Would this work just fine?"

To my knowledge (discounting number of VLANs - more on that next), yes, I would expect it to work just fine (I'm assuming distance latency isn't an issue, either).  (Note in my prior post I even mentioned L2 OSPF neighbors on other sides of the world.)

"Am i correct in understanding that the OSPF instance is not affected by the amount of vlans, because the routers communicate through multicast, and all is needed is to input..."

Yes and no.  OSPF, overall, on any one device, is impacted by the amount of work required to maintain it.  A DR does more "work", so possibly, more VLANs, more DR instances, more "work", can impair OSPF.  For this reason, you can prioritize, DR election selection, even fully excluding a DR election (per device/network), altogether.

Sort of like if you have multiple routers acting as HSRP gateway on multiple networks, you can "spread the load" by determining which router is the primary HSRP gateway per VLAN.  With more routers and/or networks, you have more choices how to "spread the load".  Basically, ditto for DRs.

Also, like with HSRP, you might determine what router is acting as DR for shared networks also based on the overall "power/performance" capacity of the router.

so R1-trunk (two VLAN)-R2


I make R1 DR priority have prefer than R2 for VLAN 1
I make R2 DR priority have prefer that R1 for VLAN 2

and both are same Peer !!!!!!!!!!!!!!!!
I really dont get this scenario 
as I read from CCDA 
all VLAN is passive except one. 
I really need to know where this scenario use ?

"I really dont get this scenario"

Again, if R1 and R2 share multiple networks, selecting which is DR, for the different shared networks, seems mostly to just distribute the DR load.

Personally, with modern devices, often the load difference is so small (compared to the overall capacity of the device), it likely, often, doesn't matter.  Further, within LANs, I generally don't see topologies that even need a DR.

Something I bring up from time to time, we often deal with "things" caused by physical device considerations from decades ago.  Something like OSPF's DRs might fall into that category.

MHM Cisco World
Advisor
Advisor

Am i correct in understanding that the OSPF instance is not affected by the amount of vlans, because the routers communicate through multicast, and all is needed is to input...

I don't get what instance you meaning here but,

If between two l3sw many vlan, then only one vlan need to opsf established other must config with passive, this reduce cpu utilize of sw that must handle many opsf hello message between same two l3sw.

Thank you for your replies! I am satisfied with these answers.

I'm going to start a new thread on a tangent subject regarding multicast and level 3 and level 2 seperation. Hope to see you there!

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