Tips and Concepts

OSPF: Point-to-Point Ethernet

On a /30 Ethernet interface network, OSPF will select one router to be a DR and the other as BDR. Ethernet by default is a broadcast network therefor DR selection must take place.

Quick review on OSPF neighbor establishment and route exchange:

1.
Down state - OSPF process has not stated
2.
Init state - Hello packets are send out on all OSPF interfaces.
3.
Two-way state - Hello received from neighbor. If the neighbor hello is in the same OSPF area, routers become neighbor.
4.
Exstart state - Routers determine which neighbor will become adjacent and which one start the exchange process.
5.
Exchange state - Routers exchange DBDs listing the LSA.
6.
Loading state - Routers update their's DBD and send LSR for missing or outdated LSA and SPF will computed.
7.
Full state - Routers LSDB has synchronized

The Exstart state typically take 40 seconds to 1 minute.
Just by changing the OSPF interface network type to point-to-point (ip ospf network point-to-point), DR/BDR election will not take place thereby no need for 40+ seconds wait phase (EXSTART to EXCHANGE). Router will go from two-way state to exchange, loading then full state. This could take no more than 1 seconds depending on how large you network is and speedup OSPF adjacency formation or topology restoration once the broken link is up.
Another advantage is one less LSA (LSA type 2 generated by an DR), smaller LSDB , quicker convergence and easier to troubleshoot.

Lets take a closer look at a the Outputs:

OSPF interface is default as Broadcast

The no shut were issued to bring up the interface, layer 2 goes up at 23:19:49. It took a full 40 seconds (23:20:29) before OSPF fully loaded.
Also from the Link State Database, OSPF have to maintain a Link States database for each Multi-Access or Broadcast network and its DR router.


OSPF Point-to-Point network

Next, we change the OSPF interface network type to Point-to-Point by issue the command "ip ospf network point-to-point" under the interface and see what happen.

OSPF interface has been changed to Point-to-Point network

As layer 2 goes up at 22:58:33, it took less than a second latter for OSPF fully loaded and no DR to be worried about.