OSPF - 2 - OSPF Design and LSAs

OSPF Design Terms 

A router that is actively  attached to multiple areas (that is, has at least one active interface in these areas), including the backbone area, considers itself an ABR. Autonomous System Boundary Routers(ASBR) inject routes external to OSPF into the OSPF domain.

Conceptually, an OSPF router keeps an independent and separate LSDB for each area to which it is connected. An internal router to an area has a single LSDB; an ABR has multiple separate LSDBs, one for each connected area. When computing a routing table, SPF is run in each LSDB separately, and the results are combined in a single routing table subject to OSPF path preference rules.

It is important to keep in mind that in multiarea OSPF, ABRs maintain separate per-area LSDBs and run SPF in each of them independently, and then combine the results and use them to populate per-area LSDBs with condensed information about other areas.

Using areas provides the following benefits:
  -Generally smaller per-area LSDBs, requiring less memory.
  -Faster SPF computation thanks to the sparser LSDB.
  -A link failure in one area only requires a partial SPF computation in other areas.
  -Routes can be summarized and filtered only at ABRs (and ASBRs). Having areas permits summarization, again shrinking the LSDB and improving SPF calculation performance.

The size of the LSDB on most routers should shrink. The LSDB shrinks because an ABR does not pass denser and more detailed type 1 and 2 LSAs from one area to another; instead, it passes type 3 summary LSAs.

OSPF Path Selection Process  

  -OSPF always chooses an intra-area route over an inter-area route for the same prefix, regardless of metric.
  -ABRs ignore type 3 LSAs learned in a nonbackbone area during SPF calculation, which prevents an ABR from choosing a route that goes into a nonbackbone area and then back into the backbone. (to prevent routing loops)

LSA Types  

An important fact concerning all LSA types is that only a router that has originated a particular LSA is allowed to modify it or withdraw it. Other routers must  process and flood this LSA within its defined flooding scope if they recognize the LSA’s type and contents, but they must not ever change its contents, block it or drop it before its maximum lifetime has expired. This requirement makes sure that all routers in an area have the same LSDB contents and have a consistent view of the network.
As a result, summarization and route filtering can be done in a very limited fashion, unlike in distance vector protocols, where summarization and route filtering can be performed at any point in the network.

Transit network: A network over which two or more OSPF routers have become neighbors and elected a DR so that traffic can transit from one to the other. An exception to this rule is a point-to-point interconnection between two routers.
Stub network: A subnet on which a router has not formed any neighbor relationships.  

LSA Types 1 and 2  

Each router creates and floods a type 1 LSA for itself. These LSAs describe the router, its interfaces (in that area), and a list of neighboring routers (in that area) on each interface. The LSA itself is identified by a link-state ID(LSID) equal to that router’s RID.
Type 2 LSAs represent a transit subnet for which a DR has been elected. The LSID is the DR’s interface IP address on that subnet. Note that type 2 LSAs are not created for subnets on which no DR has been elected.

With all the type 1 and 2 LSAs inside an area, a router’s SPF algorithm is able to create a topological graph of the network, calculate the possible routes and finally choose the best routes. For subnets without a DR, the type 1 LSAs hold enough information for the SPF algorithm to create the math model of the topology.

In the show ip ospf database output: The Link ID column should say as Link State ID. While Link State ID is a unique identifier of an entire LSA, a Link ID is a particular entry specifically in a type 1 LSA body that describes an adjacency to a neighboring object of a router. A single type 1 LSA identified by a single Link State ID can describe several adjacencies represented by several Link ID entries. These two terms are not interchangeable.  

The show ip ospf database  command lists the LSAs in that router’s LSDB, with LSA type 1 LSAs (router LSAs) first, then type 2(network link states), continuing sequentially through the LSA types. 

LSA Type 3 and Inter-Area Costs

ABRs do not forward type 1 and 2 LSAs from one area to another. Instead, ABRs advertise type 3 LSAs into one area to represent subnets described in both the type 1 and 2 LSAs in another area. Each type 3 summary LSA describes a simple inter-area destination—the subnet, the mask, and the ABR’s cost to reach that subnet.

Routers calculate the cost for a route to a subnet defined in a type 3 LSA by adding the calculated cost to reach the ABR that created and advertised the type 3 LSA and the cost as listed in the type 3 LSA. You can see the cost of the type 3 LSA with the show ip ospf database summary link-id command, and the cost to reach the advertising ABR with the show ip ospf border-routers command, and show ip ospf statistics to list the number of SPF calculations.

Withdrawal of type 3 LSAs does not require a full SPF run. Instead, routers in these areas simply check whether there is another type 3 LSA concerning the same networks providing a backup path, and when they find there is none, they simply remove the affected networks from their routing tables.  

Type 3 summary LSAs are flooded only within the area into which they were originated by ABRs. They do not cross area boundaries. Instead, ABRs compute an internal OSPF routing table for the backbone area using all types of LSAs received in the backbone area, and for each intra-area and inter-area route, they originate a new type 3 LSA to be flooded to their attached nonbackbone areas. Two important rules about originating and processing type 3 LSAs:

  -An ABR uses only those type 3 LSAs that are received over a backbone area in its SPF calculation. Type 3 LSAs received over nonbackbone areas will be skipped during the ABR’s SPF computation, though they are stored in the ABR’s LSDB and flooded within that nonbackbone area as usual.   

  -When an ABR creates and floods type 3 LSAs to advertise networks from one area to another, only intra-area routes from nonbackbone areas are advertised into the backbone; both intra-area and inter-area routes are advertised from the backbone into nonbackbone areas.    

LSA Types 4 and 5, and External Route Types 1 and 2 

OSPF allows for two types of external routes- type 1 and 2. The type determines whether only the external metric is considered by SPF when picking the best routes (external type 2, or E2), or whether both the external and internal metrics are added together to compute the metric (external type 1, or E1). By default, Cisco routers use the E2 metric type in redistribution.  

When an ASBR injects an external route, it creates a type 5 LSA for the subnet. The LSA lists the metric and the metric type. The ASBR then floods the type 5 LSA throughout all regular areas. Other routers process the LSA depending on the metric type.

the total cost of E1 external routes is computed as the cost of reaching the ASBR advertising the network, plus the E1 cost of the external network. The path with the least total cost is used; if there are multiple such paths, use them all. The total cost of E2 external routes is immediately the E2 cost of the external network. The path with the least E2 cost is used, and in case of a tie, the path having the least cost to an advertising ASBR is used; if there are still multiple paths, use them all. If there are both E1 and E2 routes to the same external network available, the E1 is always preferred to E2.  

when an ABR then floods the type 5 LSA into another area, the ABR creates a type 4 LSA, containing the ASBR’s RID and the ABR’s metric to reach the ASBR that created the type 5 LSA. Routers in other areas use the type 4 LSA to know what ASBRs in other areas exist, what ABRs can be used to reach them, and what is the distance of each ABR to a particular ASBR. If the ASBR is in the same area as the computing router, it is computed using the type 1 and 2 LSAs in that area. If the ASBR is in a different area, the cost of reaching it is computed using the type 1 and 2 LSAs in  the computing router’s area toward an ABR, plus the cost from the ABR’s type 4 LSA toward the ASBR. Note that a type 4 LSA concerning a particular ASBR is not required in the area where the ASBR reside and hence never flooded into it.

show ip ospf database external 192.168.2.0, show ip ospf database asbr-summary, show ip ospf border-routers are useful commands.

OSPF Design in Light of LSA Types  

Stubby Areas  

Not all areas need to have knowledge about individual external networks. Stubby area is an area that does not contain an ASBR and thus does not mediate an external connectivity to the entire OSPF domain. Such an area does not really benefit from knowing about individual external networks.
If an area is configured as a stubby area, ABRs will stop advertising type 4 and 5 LSAs into this area. In addition, every internal router in a stubby area will ignore any received type 5 LSAs, and will not originate any such LSAs itself. As a result, no external networks or ASBRs will be known by any internal router in a stubby area. In addition, ABRs in a stubby area will automatically inject a default route into the area as a type 3 LSA. Stubby area mainly limits the external routes(type 4 and 5); replacing those routes with a default rotue generted by ABR. The visibility of intra-area and inter-area networks in a stubby area is not affected in any way. A stubby area can contain one or more ABRs.

All four stub area types stop type 4 and 5 LSAs from entering the area. When the name includes “totally,” type 3 LSAs are also not passed into the area by ABRs except a type 3 LSA carrying the default route, significantly reducing the size of the LSDB. If the name includes “NSSA,” it means that external routes can be redistributed into OSPF by routers inside the stubby area;

In areas that are totally stubby, non-ABRs should omit the  no-summary  keyword because the additional type 3 LSA filtering is performed only on ABRs. 

NSSA can hold an ASBR and perform external route injection. This external information is carried in type 7 LSAs to distinguish it from normal external routes in type 5 LSAs, which are still prohibited even in NSSAs. In addition, the ABR with the highest RID will perform a translation from type 7 LSA to type 5 LSA and thereby inject the external route to other areas.

The NSSA is also the only nonregular type of area into which a default route is not advertised automatically. To advertise a default route into an NSSA, ABRs must be configured with the area area-id nssa default-information-originate command. All other nonregular area types will inject a default route automatically, including totally NSSA(NSSA-TS). show ip ospf database nssa-external, show ip ospf database | begin Type-5

OSPF Path Choices That Do Not Use Cost 

Under most circumstances, when an OSPF router runs the SPF algorithm and finds more than one possible route to reach a particular subnet, the router chooses the route with the least cost.

Choosing the Best Type of Path

If there are multiple routes with different route types to reach a given subnet,the router ignores the costs and instead choose the best route based on this preference - Intra-area routes, Inter-area routes, E1/N1 routes, E2/N2 routes.

Best-Path Side Effects of ABR Loop Prevention

The other item that affects OSPF best-path selection relates to some OSPF loop-avoidance features. Inside an area, OSPF uses Link State logic, but between areas, OSPF acts as a Distance Vector (DV) protocol. OSPF uses some of the same underlying concepts of DV loop-avoidance features, including Split Horizon. From a nonbackbone area, only internal routes can be advertised into the backbone.

OSPF Path Choices That Do Not Use Cost

Under most circumstances, when an OSPF router runs the SPF algorithm and finds more than one possible route to reach a particular subnet, the router chooses the route with the least cost.

Choosing the Best Type of Path

If there are multiple routes with different route types to reach a given subnet,the router ignores the costs and instead choose the best route based on this preference - Intra-area routes, Inter-area routes, E1/N1 routes, E2/N2 routes.


Best-Path Side Effects of ABR Loop Prevention

The other item that affects OSPF best-path selection relates to some OSPF loop-avoidance features. Inside an area, OSPF uses Link State logic, but between areas, OSPF acts as a Distance Vector (DV) protocol. OSPF uses some of the same underlying concepts of DV loop-avoidance features, including Split Horizon. From a nonbackbone area, only internal routes can be advertised into the backbone. ABR - genereate Type 3 LSAs for all inter/intra area routes from backbone and install them in non-backbone; generate Type 3 LSAs only for intra area routes from non-backbone to backbone. An ABR ignores type 3 LSAs from another ABR over nonbackbone area. These routing decisions can result in arguably suboptimal routes, and even asymmetric routes.