Single-Area OSPF

If an area becomes too big, the following issues must be addressed

* Large routing table - OSPF does not perform route summarization by default. If the routes are not summarized, the routing table can become very large, depending on the size of the network.

* Large link-state database (LSDB) - Because the LSDB covers the topology of the entire network, each router must maintain an entry for every network in the area, even if not every route is selected for the routing table.

Multiarea OSPF

Requires a hierarchical network design. The main area is called the backbone area (area 0) and all other areas must connect to the backbone area.

OSPF have these advantages:

1. Smaller routing tables - There are fewer routing table entries as network addresses can be summarized between areas.

Ej: R1 summarizes the routes from area 1 to area 0 and R2 summarizes the routes from area 51 to area 0. R1 and R2 also propagate a default static route to area 1 and area 51.

2. Reduced link-state update overhead - Minimizes processing and memory requirements, because there are fewer routers exchanging LSAs

Multiarea OSPF

With hierarchical routing, routing still occurs between the areas (interarea routing); while many of the tedious routing operations, such as recalculating the database, are kept within an area.

OSPF have these advantages:

3. Reduced frequency of SPF calculations - Localizes impact of a topology change within an area. For instance, it minimizes routing update impact, because LSA flooding stops at the area boundary.

Ej: Only the routers in area 51 exchange LSAs and rerun the SPF algorithm for this event. R1 does not receive LSAs from area 51 and does not recalculate the SPF algorithm.

OSPF Two-Layer Area Hierarchy

Multiarea OSPF is implemented in a two-layer area hierarchy:

1. Backbone (Transit) area - An OSPF area whose primary function is the fast and efficient movement of IP packets.

Backbone areas interconnect with other OSPF area types. The backbone area is also called OSPF area 0. Hierarchical networking defines area 0 as the core to which all other areas directly connect.

Note: A regular area can have a number of subtypes, including a standard area, stub area, totally stubby area, and not-so-stubby area (NSSA). Stub, totally stubby, and NSSAs are beyond the scope of this chapter.

OSPF Two-Layer Area Hierarchy

2. Regular (Non-backbone) area - Connects users and resources. Regular areas are usually set up along functional or geographical groupings. By default, a regular area does not allow traffic from another area to use its links to reach other areas

** The optimal number of routers per area varies based on factors such as network stability, but Cisco recommends the following guidelines:

a. An area should have no more than 50 routers.

b. A router should not be in more than three areas.

c. Any single router should not have more than 60 neighbors.

Types of OSPF Routers

The OSPF routers are categorized based on the function they perform in the routing domain.

There are four different types of OSPF routers:

1. Internal router – This is a router that has all of its interfaces in the same area. All internal routers in an area have identical LSDBs.

Types of OSPF Routers

2. Backbone router – This is a router in the backbone area. Generally, the backbone area is set to area 0

** A router can be classified as more than one router type. For example, if a router connects to area 0 and area 1, and in addition, maintains routing information for another, non-OSPF network, it falls under three different classifications: a backbone router, an ABR, and an ASBR.

Types of OSPF Routers

3. Area Border Router (ABR) – This is a router that has interfaces attached to multiple areas. It must maintain separate LSDBs for each area it is connected to, and can route between areas. ABRs are exit points for the area, which means that routing information destined for another area can get there only via the ABR of the local area.

ABRs can be configured to summarize the routing information from the LSDBs of their attached areas. ABRs distribute the routing information into the backbone.

Types of OSPF Routers

4. Autonomous System Boundary Router (ASBR) – This is a router that has at least one interface attached to an external internetwork (another autonomous system), such as a non-OSPF network.

** An ASBR can import non-OSPF network information to the OSPF network, and vice versa, using a process called route redistribution.

** Redistribution in multiarea OSPF occurs when an ASBR connects different routing domains (e.g., EIGRP and OSPF) and configures them to exchange and advertise routing information between those routing domains.

OSPF LSA Types

LSAs are the building blocks of the OSPF LSDB. Individually, they act as database records and provide specific OSPF network details. In combination, they describe the entire topology of an OSPF network or area.

The RFCs for OSPF currently specify up to 11 different LSA types

However, any implementation of multiarea OSPF must support the first five LSAs: LSA 1 to LSA 5 (Most Common). The focus of this topic is on these first five LSAs.

OSPF LSA Type 1 - Router LSA

* All routers advertise their directly connected OSPF-enabled links in a type 1 LSA and forward their network information to OSPF neighbors. The LSA contains a list of the directly connected interfaces, link types, and link states.

* Type 1 LSAs are also referred to as router link entries.

* Type 1 LSAs are flooded only within the area in which they originated. ABRs subsequently advertise the networks learned from the type 1 LSAs to other areas as type 3 LSAs.

* The type 1 LSA link ID is identified by the router ID of the originating router.

OSPF LSA Type 2 - Network LSA

A type 2 LSA only exists for multiaccess and non-broadcast multiaccess (NBMA) networks where there is a DR elected and at least two routers on the multiaccess segment.

The type 2 LSA contains the router ID and IP address of the DR, along with the router ID of all other routers on the multiaccess segment. A type 2 LSA is created for every multiaccess network in the area.

OSPF LSA Type 2 - Network LSA

Ej: ABR1 is the DR for the Ethernet network in area 1. It generates the type 2 LSA and forwards it into area 1. ABR2 is the DR for the multiaccess network in area 0. There are no multiaccess networks in area 2 and therefore, no type 2 LSAs are ever propagated in that area.

* The link-state ID for a network LSA is the IP interface address of the DR that advertises it.

* Only a DR generates a type 2 LSA.

OSPF LSA Type 3 - Summary LSAs

* Type 3 LSAs are used by ABRs to advertise networks from other areas. ABRs collect type 1 LSAs in the LSDB. After an OSPF area has converged, the ABR creates a type 3 LSA for each of its learned OSPF networks. Therefore, an ABR with many OSPF routes must create type 3 LSAs for each network

OSPF LSA Type 3 - Summary LSAs

Ej: ABR1 and ABR2 floods type 3 LSAs from one area to other areas. The ABRs propagate the type 3 LSAs into other areas. In a large OSPF deployment with many networks, propagating type 3 LSAs can cause significant flooding problems. For this reason, it is strongly recommended that manual route summarization be configured on the ABR.

* By default, routes are not summarized.

OSPF LSA Type 4 - Summary LSAs

** Type 4 and type 5 LSAs are used collectively to identify an ASBR and advertise external networks into an OSPF routing domain.

A type 4 summary LSA is generated by an ABR only when an ASBR exists within an area. A type 4 LSA identifies the ASBR and provides a route to it.

Ej: the ASBR sends a type 1 LSA, identifying itself as an ASBR. The LSA includes a special bit known as the external bit (e bit) that is used to identify the router as an ASBR. When ABR1 receives the type 1 LSA, it notices the e bit, it builds a type 4 LSA, and then floods the type 4 LSA to the backbone (area 0). Subsequent ABRs flood the type 4 LSA into other areas.

OSPF LSA Type 5 - AS External LSA

* Type 5 external LSAs describe routes to networks outside the OSPF autonomous system. Type 5 LSAs are originated by the ASBR and are flooded to the entire autonomous system.

* Type 5 LSAs are also referred to as autonomous system external LSA entries.

OSPF LSA Type 5 - AS External LSA

Ej: the ASBR generates type 5 LSAs for each of its external routes and floods it into the area.

Subsequent ABRs also flood the type 5 LSA into other areas. Routers in other areas use the information from the type 4 LSA to reach the external routes.

OSPF Routing Table Entries - descriptors

* O - Router (type 1) and network (type 2) LSAs describe the details within an area. The routing table reflects this link-state information with a designation of O, meaning that the route is intra-area.

* O IA - When an ABR receives summary LSAs, it adds them to its LSDB and regenerates them into the local area. When an ABR receives external LSAs, it adds them to its LSDB and floods them into the area. The internal routers then assimilate the information into their databases. Summary LSAs appear in the routing table as IA (interarea routes).

* O E1 or O E2 - External LSAs appear in the routing table marked as external type 1 (E1) or external type 2 (E2) routes.

OSPF Routing Table Entries - descriptors

Ej: an IPv6 routing table with OSPF router, interarea, and external routing table entries.

OSPF Route Calculation

The order in which the best paths are calculated is as follows:

1. Calculate intra-area OSPF routes.

2. Calculate best path to interarea OSPF routes.

3. Calculate best path route to external non-OSPF networks.

Implementing Multiarea OSPF - 4 steps

Step 1. Gather the network requirements and parameters - This includes determining the number of host and network devices, the IP addressing scheme (if already implemented), the size of the routing domain, the size of the routing tables, the risk of topology changes, and other network characteristics.

Step 2. Define the OSPF parametersIf multiarea OSPF is selected, there are several considerations the network administrator must take into account while determining the OSPF parameters, to include:

* IP addressing plan - This governs how OSPF can be deployed and how well the OSPF deployment might scale. A detailed IP addressing plan, along with the IP subnetting information, must be created.

Configuring Multiarea OSPF

A router simply becomes an ABR when it has two network statements in different areas.

Ej: R1 is assigned the router ID 1.1.1.1. This example enables OSPF on the two LAN interfaces in area 1. The serial interface is configured as part of OSPF area 0. Because R1 has interfaces connected to two different areas, it is an ABR.

Configuring Multiarea OSPFv3

There are no special commands required. A router simply becomes an ABR when it has two interfaces in different areas.

Ej: R1 is assigned the router ID 1.1.1.1. The example also enables OSPF on the LAN interface in area 1 and the serial interface in area 0. Because R1 has interfaces connected to two different areas, it becomes an ABR.

OSPF Route Summarization

Normally, type 1 and type 2 LSAs are generated inside each area, translated into type 3 LSAs, and sent to other areas. If area 1 had 30 networks to advertise, then 30 type 3 LSAs would be forwarded into the backbone.

With route summarization, the ABR consolidates the 30 networks into one of two advertisements.

OSPF Route Summarization

Summarization also helps increase the network’s stability, because it reduces unnecessary LSA flooding. This directly affects the amount of bandwidth, CPU, and memory resources consumed by the OSPF routing process. Without route summarization, every specific-link LSA is propagated into the OSPF backbone and beyond, causing unnecessary network traffic and router overhead.

Ej: a network link on R1a fails. R1a sends an LSA to R1. However, R1 does not propagate the update, because it has a summary route configured. Specific-link LSA flooding outside the area does not occur.

Interarea and External Route Summarization

In OSPF, summarization can only be configured on ABRs or ASBRs. Instead of advertising many specific networks, the ABR routers and ASBR routers advertise a summary route. ABR routers summarize type 3 LSAs and ASBR routers summarize type 5 LSAs.

1. Interarea route summarization - Interarea route summarization occurs on ABRs and applies to routes from within each area. It does not apply to external routes injected into OSPF via redistribution. To perform effective interarea route summarization, network addresses within areas should be assigned contiguously so that these addresses can be summarized into a minimal number of summary addresses.

Interarea and External Route Summarization

2. External route summarization - External route summarization is specific to external routes that are injected into OSPF via route redistribution. Again, it is important to ensure the contiguity of the external address ranges that are being summarized. Generally, only ASBRs summarize external routes.

Ej: EIGRP external routes are summarized by ASBR R2 in a single LSA and sent to R1 and R3.

Note: External route summarization is configured on ASBRs using the:

"summary-address (address mask)" router configuration mode command.

Interarea Route Summarization

* Summarization of internal routes for Interarea distribution can only be accomplished on ABRs

A summary route is generated if at least one subnet within the area falls in the summary address range. The summarized route metric is equal to the lowest cost of all subnets within the summary address range.

** Note: An ABR can only summarize routes that are within the areas connected to the ABR.

Calculating the Summary Route

Step 1. List the networks in binary format.

Step 2. Count the number of far left matching bits to determine the mask for the summary route.

Step 3. Copy the matching bits and then add zero bits to determine the summarized network address.

Ej: the summary address matches four networks although only two networks exist.

Configuring Interarea Route Summarization

To manually configure interarea route summarization on an ABR, use the "area (area-id) range (address) (mask)" router configuration mode command. This instructs the ABR to summarize routes for a specific area before injecting them into a different area, via the backbone as type 3 summary LSAs.

Note: In OSPFv3, the command is identical except for the IPv6 network address. The command syntax for OSPFv3 is:

"area (area-id) range (prefix/prefix-length)"

Configuring Interarea Route Summarization

Ej: Figure 3 displays the OSPF routes from the IPv4 routing table of R1. Notice how a new entry has appeared with a Null0 exit interface. The Cisco IOS automatically creates a summary route to the Null0 interface when manual summarization is configured to prevent routing loops. A packet sent to a null interface is dropped.

** For example, assume R1 received a packet destined for 10.1.0.10. Although it would match the R1 summary route, R1 does not have another valid route in area 1. Therefore, R1 would refer to the routing table for the next longest match, which would be the Null0 entry. The packet would get forwarded to the Null0 interface and dropped. This prevents the router from forwarding the packet to a default route and possibly creating a routing loop.

Configuring Interarea Route Summarization

Ej: displays the updated R3 routing table. Notice how there is now only one interarea entry going to the summary route 10.1.0.0/22.

Although this example only reduced the routing table by one entry, summarization could be implemented to summarize many networks. This would reduce the size of routing tables.

Verifying Multiarea OSPF

The same verification commands used to verify single-area OSPF also can be used to verify the multiarea OSPF topology in the figure:

show ip ospf neighbor

show ip ospf

show ip ospf interface

Verify General Multiarea OSPF Settings

Ej: ? command to verify the OSPF status. The output of the command reveals which routing protocols are configured on a router. It also includes routing protocol specifics such as the router ID, number of areas in the router, and networks included within the routing protocol configuration.

Verify General Multiarea OSPF Settings

Ej: ? Command to display concise OSPF-related information of OSPF-enabled interfaces. This command reveals useful information, such as the OSPF process ID that the interface is assigned to, the area that the interfaces are in, and the cost of the interface.

Verify the OSPF Routes

Ej: ? displays the routing table of R1.

Notice how the O IA entries in the routing table identify networks learned from other areas. Specifically, O represents OSPF routes, and IA represents interarea, which means that the route originated from another area.