How OSPF understand the shared topology towards DR/BDR election

[guhanece]

Symptoms:

With the attached diagram which has L3 point-to-point connectivity with help  switches - why DR/BDR election happens here, Inferred below details:

Router0#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.20.1 1 FULL/DR 00:00:31 11.0.0.2 GigabitEthernet0/0/1
Router0#

Router1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.10.1 1 FULL/BDR 00:00:37 11.0.0.1 GigabitEthernet0/0/1
Router1#

Diagnosis:

An important distinction to make when designing or analyzing OSPF (Open Shortest Path First) networks.

In the topology you've shown, although there are point-to-point connections between routers and switches, the network type as interpreted by OSPF depends on the interface type and configuration, not just the physical layout.

Solution:

1)You're seeing DR/BDR elections because OSPF is using the default broadcast network type on Ethernet interfaces.

2)If you want no DR/BDR (and direct adjacency between routers), configure those interfaces as OSPF point-to-point.

3)Switches in this setup act just as Layer 2 media, but OSPF sees the Ethernet as broadcast-capable unless told otherwise.

[Rich R]

> 2)If you want no DR/BDR (and direct adjacency between routers), configure those interfaces as OSPF point-to-point.

@guhanece if you're advising people to do that then you should at least explain how to configure that - which is:
ip ospf network point-to-point
configured on the IP interface.
And provide some reference material for people to refer to:
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16-8/iro-xe-16-8-book/iro-multi-area-adj-xe.html
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/118879-configure-ospf-00.html
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/command/iro-cr-book/m_ospf-i1.html#wp3564440872

[Joseph W. Doherty]

In addition to @Rich R 's comment, I too have a few critiques.

Firstly, although I'm sure the intent of this document was intended to address a default behavior of OSPF on multi-access (in this example case, Ethernet) media, which might not be the optimal choice, but when "Symptoms", "Diagnosis" and "Solution" categories are emphasized, it would infer we're dealing with some kind of problem. Again, there's no real problem, as OSPF will work fine, in this case, with adjacent neighbors using DR/BDR, again the default, and expected, behavior.  (As an aside, I believe one of our VIPs, @MHM Cisco World , actually prefers using, in these cases, DR/BDR, in cases where an OSPF p2p might be configured.)

Second, the posted topology really doesn't, I believe, make clear there's a (logical) p2p topology between the two routers, transiting switches.  Certainly there can be, but the switches, implicitly, support multi devices connecting to the L2 domain in addition to these two routers.

If, perhaps, an actual physical p2p Ethernet connection was used between two routers, it would better highlight, that because Ethernet is assumed to be multi-access, OSPF would default to using DR/BDR adjacency.

Or, perhaps both physical topologies should be shown, i.e. a physical p2p and connection via a switch.  (Also the second switch, I don't believe, doesn't seem to contribute to this example.)

In the case where you do setup OSPF p2p on a topology that physically doesn't insure p2p, you could also mention, if using just two routers, they can be defined for p2p, but if a third router was added, it wouldn't automatically be able to form an adjacency with the pre-existing other two.

BTW, "3) Switches in this setup act just as Layer 2 media, but OSPF sees the Ethernet as broadcast-capable unless told otherwise." they do, but that's immaterial to the two routers.  What matters is "1)You're seeing DR/BDR elections because OSPF is using the default broadcast network type on Ethernet interfaces."

"2)If you want no DR/BDR (and direct adjacency between routers), configure those interfaces as OSPF point-to-point."  That's fine, if you're sure that's what you want.  Again, OSPF will work fine using DR/BDR.  There are a few advantages, though, of using p2p, which you might note.