Ospf neighbor

From the ciscopress book for ICND in the chapter on Ospf it says 'on an interface that does not use a DR, Ospf updates can be sent to all neighbors on that interface'
I thought the whole point of using a DR was to reduce ospf traffic, why would 2 (non-DR)neighbors be sending each other updates?

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Comments

networker050184

Not all networks call for the use of a DR. If the routers are not on the same segment or broadcast network no DR is chosen. Therefore interfaces where no DR is used updates are sent to the neighbor.

Mikdilly

networker050184 wrote:

Not all networks call for the use of a DR. If the routers are not on the same segment or broadcast network no DR is chosen. Therefore interfaces where no DR is used updates are sent to the neighbor.

So if you had 2 ospf routers separated by a switch or hub they would never use a DR even though they may be on the same subnet?

networker050184

If you only had two there wouldn't be a need for a DR. If multiple switches connected to a switch and on the same VLAN, then you would have a DR. If you have two routers connected by a point to point link, then there is no need for a DR.

Mikdilly

networker050184 wrote:

If you only had two there wouldn't be a need for a DR. If multiple switches connected to a switch and on the same VLAN, then you would have a DR. If you have two routers connected by a point to point link, then there is no need for a DR.

Did you mean multiple routers connected to a switch and on the same vlan?

networker050184

Yeah thats what I meant sorry

srg

DRs doesnt need to be chosen on links that arent multiaccess.. serial links for example.

Because serial links are just point-to-point, there can't be 3 och 7 routers talking on the same serial-line, only 2. So "all neighbors" on that interface will be just the one router on the other side.

dtlokee

Designated routers are only elected on broadcast and nonbroadcast network types. It's important to understand OSPF network types which are not tied to the type of interface. For example a point to point interface (one using HDLC, or PPP for example) will default to the OSPF network type point to point. This network type does not require a DR or BDR on the segment, but you can change the network type to "broadcast" then it will use a DR and BDR.

To see the OSPF network type of an iterface use the show ip ospf interface command.

Mikdilly

But getting back to what the text says, 'an interface that does not use a DR, updates can be sent to all neighbors on that interface'. They're making it sound like there is more than one neighbor on the interface which would seem to indicate that it's not a point-to-point link. Is it possible for an interface to not use a DR and not be a point-to point link?

Netstudent

yes, in frame-relay. You can create a full mesh frame-relay with a OSPF point-to-multipoint network type. But i'm not sure what that text is referring to.

Mikdilly

Netstudent wrote:

yes, in frame-relay. You can create a full mesh frame-relay with a OSPF point-to-multipoint network type. But i'm not sure what that text is referring to.

It's the first paragraph on page 198.

Netstudent

ok then yes, the example i described is one example of how this can happen.