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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.

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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


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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