OSPF notes

posted 12 Apr 2016, 13:34 by DR Labs   [ updated 20 Jun 2016, 03:54 by Donald Ross ]
OSPF open shortest path first  (link state protocol)

OSPF network types
Point to Multi Point
Virtual links

Neighbour formation
Network command 
Interface command
Passive command

Example configuration -
Router OSPF 1
network (wildcard)
network area 0 (interface only)
network area 0 (any)

OSPF states

Init - hello's  (nothing coming back) This state specifies that the router has received a hello packet from its neighbor, but the receiving router's ID was not included in the hello packet. When a router receives a hello packet from a neighbor, it should list the sender's router ID in its hello packet as an acknowledgement that it received a valid hello packet.

2 way - (see hello's coming back)  This state designates that bi-directional communication has been established between two routers. Bi-directional means that each router has seen the other's hello packet.

Exstart - (negoiate exchange information) Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR.

Exchange - (exchanging information)  OSPF routers exchange database descriptor (DBD) packets. Database descriptors contain link-state advertisement (LSA) headers only and describe the contents of the entire link-state database.

Loading - the actual exchange of link state information occurs. Based on the information provided by the DBDs, routers send link-state request packets.

Full -  In this state, routers are fully adjacent with each other. All the router and network LSAs are exchanged and the routers' databases are fully synchronized.


Passive interface default - no interface will send OSPF hello's

Packet Types
Link state database
Route table 

type 1 - hello
type 2 - DBD (databse description)
type 3 - LSR (link state request
type 4 - LSU (ink state update)
type 5 - LSAck (Link state ACK)

Overview of OSPF LSA Types

We know that Link State Advertisements (LSA) are the life blood of an OSPF network. The flooding of these updates (and the requests for this information) allow the OSPF network to create a map of the network. This of course occurs with a little help from Dijkstra’s Shortest Path First Algorithm. But not all OSPF LSA’s are created equal. 

The Router (Type 1) LSA
We begin with what many call the “fundamental” or “building block” Link State Advertisement. The Type 1 LSA (also known as the Router LSA) is flooded within an area. It describes the interfaces of the local router that are participating in OSPF and the neighbors the local OSPF speaker has established.

The Network (Type 2) LSA
Do you remember how OSPF functions on an Ethernet (broadcast) segment? It elects a Designated Router (DR) and Backup Designated Router (BDR) in order to reduce the number of adjacencies that must be formed and the chaos that would result from a full mesh of these relationships. Well, the Type 2 LSA is sent by the Designated Router into the local area. This LSA describes all of the routers that are attached to that Ethernet segment.

The Summary (Type 3) LSA
Ready for a big difference with this LSA type? Recall that your Type 1 and Type 2 LSAs are sent within an area. We call these intra-area LSAs. Now it is time for the first of our inter-area LSAs. The Summary (Type 3) LSA is used for advertising prefixes learned from the Type 1 and Type 2 LSAs into a different area. Do you recall what device would send such an LSA? Sure, it would be the Area Border Router that separates areas.
So let’s say we have an area design like this – AREA 1-AREA 0-AREA 2. The Area 1 ABR would send the Type 3 LSAs into Area 0. It’s ABR into Area 2 would send these Type 3 LSAs into that area to provide full reachability in the OSPF domain. The Type 3 LSAs remain Type 3 LSAs during this journey, it is just OSPF costs and advertising router details that change in the advertisements. Notice also that in this example we are describing a multi area OSPF design that is not using any special area types like Stub or Totally Stubby.

The ASBR Summary (Type4) LSA
Do you recall the very special OSPF router that brings in routes from another domain (like an EIGRP domain)? It is the Autonomous System Boundary Router. In order to inform routers in different areas about the existence of this special router, the Type 4 LSA is used. This Summary LSA provides the router ID of the ASBR. So once again, the Area Border Router is responsible for shooting this information into the next area and we have another example of an inter-area LSA.

The External (Type 5) LSA
So the ASBR is the device that is brining in prefixes from other routing domains. The Type 4 LSA describes this device. But what LSA is used for the actual prefixes that are coming in from the other domain? Yes, you guessed it, it is the Type 5 LSA. The OSPF ASBR creates these LSAs and they are sent to the Area Border Routers for dissemination into the other areas. Remember, this might change if we are using special area types.

The NSSA External (Type 7) LSA
Remember that in OPSF there is a VERY special area type called a Not So Stubby Area. This area can act stub, but it can also bring in external prefixes from an ASBR. You guessed it, these prefixes are sent as Type 7 LSAs. When an ABR gets these Type 7 LSAs, it sends them alone in to the other areas as a Type 5 LSA. So the Type 7 designation is just for that very special NSSA area functionality.

OSPF LSA Types and Areas
  • Router (Type 1)
  • Network (Type 2)
  • Summary (Type 3)
  • ASBR Summary (Type 4)
  • External (Type 5)
  • NSSA External (Type 7)

If you are even slightly fuzzy on what these different LSAs are used for in OSPF, please quickly go over that previous post.

The purpose of this post if for us to discuss how these LSAs will be impacted by a multi area area design, especially one that might include special areas. What is wonderful about this exercise is the fact that it allows us to review what these special areas are for, and gives us a richer understand of exactly how they function. Of course, this is from the automatic filtering of certain LSAs from certain areas.

OSPF LSAs and Standard Areas

Think about an area attached to the backbone area of There are Type 1 LSAs flooding in this area If we have broadcast segments, we also have Type 2 LSAs circulating in the area. The Area Border Router is sending LSA Type 3s into the backbone to summarize the prefix information in area It is also taking in this information from the backbone for other areas that might exist. If there is an ASBR out there in the domain somewhere, our area will receive Type 4 and Type 5 LSAs in order to know the location of this ASBR and the prefixes it is sharing with us. Whew! That is a lot going on. This is precisely why we have the special area types!

OSPF LSAs and the Stub Area

What is it that we want to accomplish with a stub area? We do not want to hear about those prefixes that are external to our OSPF domain. Remember what those were? Sure, they are the Type 5 LSAs. In fact, we do not even want to hear about those Type 4 LSAs that are used to call out the ASBR in the network. So the stub area is chock full of Type 1, Type 2, and Type 3 LSAs. In fact, how would this area get to one of those external prefixes it is needed to? We typically use a very special Type 3 LSA for this. This LSA represents the default route ( It is this handy little route that allow devices in this area to get to all of those externals, in fact, to get to any prefix not specifically defined in the Routing Information Base.

OSPF LSAs and the Totally Stubby Area

Ok, with this area we want very little inside it right? Sure. So it makes sense that we are blocking those Type 4 and Type 5 once again, but now we are even blocking the Type 3 LSAs that are describing prefix information from other areas WITHIN our OSPF domain. There needs to be one big exception, however. We need a Type 3 LSA for a default route so we can actually get to other prefixes in our out of our domain.

OSPF LSAs and the Not So Stubby Area and the Totally Not So Stubby Area

Remember, the Not So Stubby Area needs to have those Type 7 LSAs. These Type 7 permit the proliferation of those external prefixes that are entering your OSPF domain thanks to this NSSA area you created. Obviously this area also has the Type 1, Type 2, and Type 3 inside it. Type 4 and Type 5 will be blocked from entering this area as you would expect. In both Juniper and Cisco environments, you can also create a Totally Not So Stubby Area by restricting Type 3s from this area.