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4.03.2013

OSPF Network Types Part 2 - Point-to-Point


The name pretty much explains it all. P2P network types are used when connecting two OSPF routers who share a directly connected segment. Here are the characteristics of the P2P network type:
  • Default for serial interfaces (HDLC or PPP)
  • Neighbor discovery
  • Multicast AllSPFRouters (224.0.0.5) used for all neighbor communications
  • No DR/BDR election (thus no Type 2 LSAs)
  • Adjacencies are formed as long as HELLO parameters match
  • Hello/Dead Timers 10/40 seconds
  • Cisco proprietary

     

Basic Configuration

We have a basic setup with three routers each directly connected to their neighbor. With this example each router is connected with FastEthernet links which by default OSPF uses the BROADCAST network type since they are Ethernet segments. 

We have a simple OSPF configuration on all three routers 


From R2 we can verify that an adjacency has formed with both R1 and R3. Notice how both neighbors are in the FULL/BDR state? This means both links held a DR/BDR election.


Fine no big deal right?

Well for a small network like this its not, but its inefficient and can cause large amounts of overhead in larger networks. Lets see how

First, we should verify the network type of the interfaces. Show ip ospf interfaces along with show ip ospf database router [link-ID] will give us this information.



Yep just as we thought. The Network Type listed in line 4 list as BROADCAST. So what does that mean for OSPF? Lets look at the OSPF database (aka LSDB) and find out.



The first line of show ip ospf database list the local OSPF RID and what process this LSDB is running under. The first section of routes listed as "Router Link States" are the Type 1 LSAs.
The Second section "Net Link States" is the Type 2 LSAs generated by the DR. These LSAs describe the network segments known by the DR.

Since these links only have two routers on the segment, all traffic from the rest of the network will be transient through the links. No on cares about the P2P segments themselves, they just want to know how to get to the other side. In a P2P situation, the Type 2 LSAs do not need to populate the LSDB because they do not provide anything useful that the Type 1 LSAs don't already provide. So, off with their heads!!!

If we configure the links as P2P links it should clean this up since P2P network types do not elect a DR and thus do not generate Type 2 LSAs. To configure the network type for OSPF interfaces use the ip ospf network interface command.

Let's have a look and see if I'm right. To save space I will only show the configuration of R1 since the configuration is the same on all three routers.


NOTE - Once the network type is changed on the interface OSPF bounces the adjacency on the effected interface. This then allows OSPF to repopulate the LSDB, thus causing a short disruption or outage. Lets hope the CEO is not streaming his cat videos on YouTube right now...

First, make sure the network type was change to POINT-TO-POINT in the show ip ospf interface command.



Yay its there!! Take a look at the show ip ospf neighbor command and see how it changed.



Notice how the state is FULL but does not have anything for DR election? On to the LSDB.



Look at that! just like we thought. The LSDB is cut in half by removing all the Type 2 LSAs from the P2P links.

Now, on to loopbacks!