DR/BDR Ethernet and non-Ethernet

Magic Johnson

Just so I've got this right:

On a serial link say, two connected routers, the DR/BDR is per link, not per area?

So say you had 4 routers, Router 1 and 3 directly connected remote office routers to router 1, there would be 3 BDRs and one DR in this area (assuming I set the R-ID on R1 to be higher than the other routers and left priority as default).

On ethernet, if there are lots of connected routers in an area (not directly) then there is only one DR and one DBR?

So say you had 8 routers, some directly connected to R1, others connected to other routers. There will be only one DR and one BDR in this topology (again assuming R1 has highest R-ID and priorities are default).

I want to clarify here first in layman's terms as I remember looking stuff up on the web and confusing myself even further during ICND1.

Cheers.

SecurityThroughObscurity

Point-to-point, point-to-multipoint, point-to-multipoint non-broadcast - no dr/bdr election.
broadcast, non-broadcast - dr/bdr election.

Magic Johnson

SecurityThroughObscurity wrote: »

Point-to-point, point-to-multipoint, point-to-multipoint non-broadcast - no dr/bdr election.
broadcast, non-broadcast - dr/bdr election.

Can you clarify 'dr/bdr election'?

Odom says there are still dr and bdr on those links, surely that means some election takes place?

SecurityThroughObscurity

It's about network type.

dr/bdr election takes place per broadcast domain. (assumed all your routers use an ethernet connection).

what network type do you use? is it multi-access or point-to-point?

Magic Johnson

SecurityThroughObscurity wrote: »

It's a network type.

dr/bdr election takes place per broadcast domain. (assumed all your routers use an ethernet connection).

what network type do you use? is it multi-access or point-to-point?

AH I think I get this. So the only reason a non-broadcast link needs a DR and BDR is for type 2 LSA purposes?

SecurityThroughObscurity

check this out OSPF network types - Packet Life

SecurityThroughObscurity

difference between broadcast and non-broadcast is that you need to manually configure neighbors.
Type 2 LSA would be generated only in dr/bdr environment.

Magic Johnson

I feel that's a bit over my head at the moment, I've not touched WAN techs yet to that depth.

For a PPP link, there will be a DR and BDR? From what you are saying that shouldn't be the case?

SecurityThroughObscurity

<<Odom says there are still dr and bdr on those links, surely that means some election takes place?

look.
R1:
int fa1/0
ip ospf network point-to-point
R2:
int fa1/0
ip ospf network point-to-point

sh ip ospf ne:

Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/ - 00:00:33 192.168.34.3 FastEthernet1/0

Neighbor ID Pri State Dead Time Address Interface
4.4.4.4 0 FULL/ - 00:00:39 192.168.34.4 FastEthernet1/0

Magic Johnson

See there's no mention of that command in the book. Is that outside the scope of CCNA?

Ah, I see my confusion. GRR. In the book, Odom uses an Ethernet WAN link not a PPP. D'oh!

SecurityThroughObscurity

R1:
int fa1/0
ip ospf network broadcast
R2:
int fa1/0
ip ospf network broadcast

Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 1 FULL/DR 00:00:38 192.168.34.3 FastEthernet1/0

Neighbor ID Pri State Dead Time Address Interface
4.4.4.4 1 FULL/BDR 00:00:39 192.168.34.4 FastEthernet1/0


forget about wan/lan.
just think about network type of ospf interface.

Magic Johnson

Yes thanks fella, I mis-saw the fact that the serial link was actually an ethernet WAN link. That was really confusing me! Need to pay attention a bit more.

Thanks for your help!

Magic Johnson

"forget about wan/lan.
just think about network type of ospf interface."

This is now burned in to my brain. Thanks.

tomtom1

And also note, there is always 1 DR and 1 BDR on a network segment. All routers with the ospf priority set to one (1) or higher take place in the DR / BDR election. If the OSPF priority is a tie (default) then the RID (router ID) of the OSPF enabled routers is the deciding factor.