Tips and Concepts

Tips from the field

 
 OSPF Point-to-Point Ethernet         Tips and Concepts

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 will 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Loading state - Routers update their's DBD and send LSR for missing or outdated LSA and SPF will computed.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 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. This will speedup OSPF adjacency formation or topology restoration once the broken link is fixed.

Another advantage is one less LSA (LSA type 2 generated by an DR), smaller LSDB , quicker convergence and easier to troubleshoot.

 
   
Optimize OSPF /30 network  
Layer-2 Resolving  
Data Encapsulation  
"Fish-Mo" of AD  
Layer-2 Redundancy  
Using /31 Subnet  
FSMO: Inf Master and GC  
   
   
   
   

Lets take a closer look at a the Outputs:

Ethernet Broadcast


As you can see from the time no shut on the fastethernet, layer 1 goes up at 23:19:48 and layer 2 goes up one second latter at 23:19:49. It took till 23:20:29 (a full 40 seconds) before OSPF fully loaded.
Also from the Link State Database, OSPF maintain a Net Link States database for each Multi-Access or Broadcast network and its DR router.

Point-to-Point

Now let take a look at Point-toPont network. From the time layer 2 goes up at 22:58:33, It took less than a second for OSPF fully loaded and no DR to be worried about.