OSPF Area and LSA Types Link state advertisement

OSPF Area and LSA Types

There are different types of OSPF Area and LSA Types. In this chapter you will learn all types of OSPF areas and Link state advertisement LSA.

Why Multiarea OSPF ?

Single area OSPF is useful in smaller networks , where the network of links between routers is simple and routes to individual destinations are easily deduced.
However, if an area grows too large, the following problems should be resolved immediately:

• Extensive routing tables : OSPF does not summarize routes by default. If the routes are not summarized, the routing table becomes very large, depending on the size of the network.
• Very large link state (LSDB) databases : because the LSDB covers the topology of the entire network, each router must maintain an entry for each network in the area, even if not all routes are selected for the table routing
• Frequent calculations of the SPF algorithm : in large networks, modifications are inevitable, so routers spend many CPU cycles recalculating the SPF algorithm and updating the routing table.

To make OSPF more efficient and scalable, this protocol supports hierarchical routing through areas. An OSPF area is a group of routers that share the same link state information in the link state databases.

Multi-area OSPF

When a large OSPF area is divided into smaller areas, this is called " multi-area OSPF ". Multi-area OSPF is useful in larger network implementations , as it reduces processing and memory overhead.
For example, every time a router receives new information about the topology, such as adding, removing or modifying a link, the router must rerun the SPF algorithm, create a new SPF tree and update the routing table .
The SPF algorithm represents a high demand for the CPU and the time it takes to perform the calculations depends on the size of the area. If there were too many routers in an area, the LSDB would be larger and the CPU load would increase. Therefore, the arrangement of routers in different areas effectively divides a potentially large database into smaller and easier to manage databases.
Multi-area OSPF requires a hierarchical network design . The main area is called “backbone network” (area 0) and the rest of the areas must be connected to it. With hierarchical routing, inter-area routing continues to occur (inter-area routing), and many of the tedious routing operations, such as recalculating the database, are stored in an area.

Advantages of multi-area OSPF

As illustrated in Image 2, the possibilities of hierarchical topology of multi-area OSPF have the following advantages:

• Smaller routing tables : there are fewer entries in the routing table, since network addresses can be summarized between areas. For example, R1 summarizes routes from area 1 to area 0 and R2 summarizes routes from area 51 to area 0. In addition, R1 and R2 propagate a predetermined static route to areas 1 and 51.
• Lower link state update overhead : minimizes processing and memory requirements, since there are fewer routers that exchange LSA.
• Lower frequency of SPF calculations: Locate the impact of a topology change within an area. For example, it minimizes the impact of a routing update, because LSA flooding stops at the area's border.

In Image, suppose a link between two internal routers in area 51 fails. Only routers in area 51 exchange LSA and rerun the SPF algorithm for this event. R1 does not receive the LSAs from area 51 and does not recalculate the SPF algorithm.

OSPF Two layer area hierarchy

The OSPF of various areas is implemented with a two-layer area hierarchy:

• Trunk (transit) network area: an OSPF area whose main function is the fast and efficient transmission of IP packets.

Trunk network areas interconnect with other types of OSPF area. In general, end users are not in a backbone network area. The backbone area is also called "OSPF area 0". In hierarchical networks, area 0 is defined as the core to which all other areas are connected directly.

Common area (not backbone network) : connect users and resources. Regular areas are generally set up in functional or geographic groups. By default, a regular area does not allow traffic from another area to use its links to reach other areas. All traffic from other areas must pass through a transit area.
Note : Common areas may have a variety of subtypes, including a standard area, an internal route area, an exclusive internal route area and a non-exclusive internal route area (NSSA).
OSPF applies this rigid two-layer area hierarchy. The underlying physical connectivity of the network must be assigned to the structure of the two-layer area, with only non-backbone areas directly connected to area 0. All traffic that is transferred from one area to the other must cross the area backbone network.
This traffic is called " inter-area traffic ".

Cisco Recommendations

 The optimal number of routers per area depends on factors such as network stability, but Cisco recommends keeping the following guidelines in mind:

• An area should not have more than 50 routers.
• A router should not be in more than three areas.
• No router must have more than 60 neighbors.

OSPF Router Types

Different types of OSPF routers control the traffic that enters and leaves the areas. OSPF routers are categorized according to the role they play in the routing domain .
There are four different types of OSPF routers:

• Internal router : it is a router whose interfaces are all in the same area. All internal routers in an area have identical LSDBs.

• Backbone router: It is a router that is in the backbone area. In general, the backbone network area is set to area 0.

Area Border router. (ABR) : It is a router whose interfaces connect to several areas. You must maintain an LSDB for each area to which you are connected; You can routing between areas. ABRs are exit points for each area. In a network of various areas, an area can have one or more ABRs.

Autonomous system Border router (ASBR) : a router that has at least one interface connected to an external internetwork (another autonomous system), for example, a non-OSPF network. An ASBR can import information from a non-OSPF network into an OSPF network, and vice versa, through a process called “route redistribution ”.

OSPF redistribution of various areas occurs when an ASBR connects different routing domains (for example, EIGRP and OSPF) and configures them to exchange and advertise routing information between said routing domains.

OSPF link State Advertisement LSA types

LSAs are the functional blocks of the OSPF LSDB . Individually, they function as database records and provide specific details of OSPF networks. Together, they describe the entire topology of an area or an OSPF network.
Currently, RFCs for OSPF specify up to 11 different types of LSAs (See table).

OSPF LSA Types Table.
LSA Type	Description
one	Router LSA
2	LSA of the network
3 and 4	Summary LSA
5	AS external LSA
6	OSPF multicast LSA
7	Defined for NSSAs
8	LSA of external attributes for the border gateway protocol (BGP)
9, 10 or 11	Opaque LSA

However, any multi-area OSPF implementation must support the first five LSAs: from LSA 1 to LSA 5. This topic focuses on these first five LSAs.
Every router link is defined as a type of LSA. The LSA comprises a link ID field that identifies, by number and netmask, the object to which the link is connected. Depending on the type, the link ID has different meanings. LSAs vary depending on how they were generated and propagated within the routing domain.
Note : OSPFv3 includes additional types of LSA.

OSPF Type 1 LSA

As shown in the figure, every router announces its OSPF links with direct connection through a type 1 LSA and forwards the information from its network to the OSPF neighbors. The LSA contains a list of interfaces with direct connection, link types and link states.

Type 1 LSAs are also referred to as "router link entries."
Type 1 LSAs only flood the area that originates them. ABRs, at the same time, announce to other areas the networks discovered from type 1 LSAs as type 3 LSAs.
The router ID that originates the area identifies the link ID of a type 1 LSA.

OSPF Type 2 LSA

A type 2 LSA only exists for networks of various accesses and networks without diverse accesses or broadcast (NBMA) where a DR is selected and at least two routers in the segment of various accesses.

• The type 2 LSA contains the router ID and DR IP address, in addition to the router ID of all other routers in the multiple access segment. A type 2 LSA is created for each multiple access network in the area.
• The purpose of a Type 2 LSA is to provide other routers with information about multiple access networks within the same area.
• The DR floods type 2 LSAs only in the area in which they originate. Type 2 LSAs are not forwarded outside the area.

Type 2 LSAs are also called "network link entries."

As shown in the figure, ABR1 is the DR of the Ethernet network of area 1. It generates LSA of type 2 and forwards them to area 1. ABR2 is the DR of the network of various accesses of area 0. There are no networks of various accesses in area 2; therefore, type 2 LSAs will never be propagated in that area.
The link status ID for a network LSA is the IP address of the DR interface that announces it.

Type 3 OSPF LSA

ABRs use Type 3 LSAs to advertise networks in other areas.
ABRs collect type 1 LSAs in the LSDB. After an OSPF area converges, the ABR creates a type 3 LSA for each recognized OSPF network. Therefore, an ABR with several OSPF routes must create a type 3 LSA for each network.
As shown in the figure, ABR1 and ABR2 propagate type 3 LSAs from one area to another.

ABRs propagate type 3 LSAs to other areas. During an important OSPF implementation with many networks, the spread of type 3 LSAs can cause significant flood problems. For this reason, it is strongly recommended that you manually configure the route summary in the ABR .

• The link status ID is set to the network number, and the mask is also announced.
• Receiving type 3 LSAs in your area does not prompt the router to execute the SPF algorithm. Routers that are advertised in type 3 LSAs are added to the router routing table or removed from it as appropriate, but the full SPF calculation is not required.

OSPF Type 4 LSA

Type 4 and type 5 LSAs are used together to identify an ASBR and advertise external networks that reach an OSPF routing domain.
The ABR generates a type 4 summary LSA only when there is an ASBR in the area. A type 4 LSA identifies the ASBR and assigns it a route . All traffic destined for an external autonomous system requires knowledge of the routing table of the ASBR that originated the external routes.

As shown in the illustration, the ASBR sends a type 1 LSA to identify itself as ASBR.
The LSA includes a special bit called an "external bit" (e bit) that is used to identify the router as an ASBR.

• When ABR1 receives the type 1 LSA, it recognizes the e bit, generates a type 4 LSA and propagates it to the backbone network (area 0).
• Subsequent ABRs propagate the type 4 LSA to other areas.
• The link status ID is set in the ASBR router ID.

OSPF type 5 LSA

External type 5 LSAs announce routes to networks outside the autonomous OSPF system . These originate in the ASBR and spread to the entire autonomous system.
Type 5 LSAs are also known as external LSA inputs of the autonomous system.

• In the figure, the ASBR generates LSA type 5 for each external route and propagates them to the area.
• Subsequent ABRs also propagate type 5 LSAs to other areas.
• Routers in other areas use LSA type 4 information to reach external routes.

During a large OSPF implementation with many networks, the spread of type 5 LSAs can cause significant flood problems. For this reason, it is strongly recommended that you manually configure the route summary in the ASBR .
The link status ID is the external network number.

OSPF Routing Table Entries

In Image 14, an example routing table is provided for a multi-area OSPF topology with a link to an external network that is not OSPF. OSPF routes in an IPv4 routing table are identified by the following descriptors:

• Or : Router (type 1) and network (type 2) LSAs describe the details within an area. The routing table reflects this link status information with the designation O, which means that the route is intra-area.
• IA O : When an ABR receives summary LSA, it adds them to its LSDB and rebuilds them in the local area.

When an ABR receives an external LSA, it adds it to its LSDB and propagates it in the area. Then, the internal routers assimilate the information in their database. Summary LSAs appear in the routing table as IA (inter-area routes).

• O E1 or O E2 : in the routing table, external LSAs are marked as external routes type 1 (E1) or external routes type 2 (E2).

In Image, an IPv6 routing table is shown with inter-area, external and OSPF router routing table entries.

SPF Calculation OSPF Router

Each router uses the SPF algorithm under the LSDB to create an SPF tree. The SPF tree is used to determine the best routes.

As shown in the figure, the order in which the best routes are calculated is as follows:

1. Every router calculates the best routes to destinations in your area (intra-area) and adds these entries to the routing table. These are type 1 and type 2 LSAs, which are indicated in the routing table with the designator "O". (one)
2. Every router calculates the best routes to other areas in the internetwork. The best routes are the inter-area route entries, or LSA type 3 and type 4, and are indicated by the routing designator "O IA". (2)
3. Every router (except those located in a form of internal routes) calculates the best routes to destinations of the external autonomous system (type 5). These are indicated by the route designator O E1 or O E2, depending on the configuration. (3)

When converging, a router communicates with any network inside or outside the autonomous OSPF system.