OSPF DR Election
DR And BDR
OSPF uses explicit acknowledgments (re-sending the LSAs), so as neighbors and adjacencies grow, the amount of OSPF traffic on a network increases.
A network with six ospf routers forming a full-mesh requires 15 adjacencies.
To mitigate the scaling problem, on broadcast segments OSPF elects a DR, and BDR, to maintain the LSDB.
The RFC calls this a “network vertex”. We can also use the term DR.
- All routers listen for hello on 224.0.0.5
- DR floods LSAs to the routers with 224.0.0.5
- DROTHER talks to the DR/BDR on 224.0.0.6
In the diagram (from the RFC), everything connects to N2, so problem solved.
**FROM**
+---+ +---+
|RT3| |RT4| |RT3|RT4|RT5|RT6|N2 |
+---+ +---+ * ------------------------
| N2 | * RT3| | | | | X |
+----------------------+ T RT4| | | | | X |
| | O RT5| | | | | X |
+---+ +---+ * RT6| | | | | X |
|RT5| |RT6| * N2| X | X | X | X | |
+---+ +---+
Broadcast or NBMA networks
See OSPF LSAs to see what the actual contents of the LSAs are.
DR
Forms full adjacencies.
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 50 FULL/BDR 00:00:31 10.0.0.2 Ethernet0/0
3.3.3.3 1 FULL/DROTHER 00:00:37 10.0.0.3 Ethernet0/0
4.4.4.4 1 FULL/DROTHER 00:00:34 10.0.0.4 Ethernet0/0
5.5.5.5 1 FULL/DROTHER 00:00:32 10.0.0.5 Ethernet0/0
6.6.6.6 1 FULL/DROTHER 00:00:31 10.0.0.6 Ethernet0/0
- First router online on the segment is the DR.
Drother
- Only forms full adjacencies with the DR, and BDR.
- When it sends LSAs, sends them to the DR/BDR via 224.0.0.6.
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 50 FULL/BDR 00:00:31 10.0.0.2 Ethernet0/0
3.3.3.3 1 FULL/DROTHER 00:00:37 10.0.0.3 Ethernet0/0
4.4.4.4 1 FULL/DROTHER 00:00:34 10.0.0.4 Ethernet0/0
5.5.5.5 1 FULL/DROTHER 00:00:32 10.0.0.5 Ethernet0/0
6.6.6.6 1 FULL/DROTHER 00:00:31 10.0.0.6 Ethernet0/0