An OSPF area is defined as a logical grouping of routers by a network
administrator. OSPF routers in any area share the same topological view
(also known as the OSPF database) of the network. The core reason that
OSPF is configured in multiple areas is to reduce routing table sizes,
which in turn reduces the topological database and CPU/memory
requirements on a router.
OSPF is not just configured in one large area, so all routers share the
same topological database. The use of multiple areas ensures that the
flooding and database management required in large OSPF networks is
reduced within each area so that the process of flooding the full
database and maintaining full network connectivity does not consume a
large portion of the CPU processing power. Every time a network change
occurs, the CPU on a router is interrupted and a new OSPF tree is
calculated. Running the shortest path first (SPF) algorithm itself is
not CPU intensive, but sending and flooding the network with new
topological information is extremely CPU intensive.
Routing tables become very large even with only 50 routers. The OSPF
database is exchanged every 30 minutes in full, and if this database is
too large, every time the exchange occurs, the amount of bandwidth used
over the network increases, which can cause severe delays in sending
user-based traffic because convergence times increase.
Considering the demands on CPU and memory along with reduced IP routing
tables, you should now have a good understanding of why OSPF requires
more than one area. In Scenario 3-2 in Chapter 3, you saw how to
configure an OSPF network that is partitioned from the backbone. All
OSPF areas must be connected to the backbone in case of network failure.
When an area cannot reside physically or logically on the backbone, a
virtual link is required. For partitioned areas, OSPF treats the area as
a separate area, and no routing information flows to the backbone;
therefore, you do not have IP connectivity.
Virtual links add a layer of complexity and might cause additional
problems when applied to large IP networks. It is best to avoid virtual
links in the real world.
When configuring a virtual link, you must be aware of the following design restrictions:
- Virtual links must be configured between two area border routers (ABRs).
- The transit area cannot be a stub area.
- The transit area must have full routing knowledge of both partitioned areas.
Table 4-1 summarizes the four OSPF area types and their functions.
Table 4-1. OSPF Router Types
| Router Type | Description |
| Internal router | This
router is within a specific area only. Internal router functions
include maintaining the OSPF database and forwarding data to other
networks. All interfaces on internal routers are in the same area. |
| Area border router (ABR) | ABRs
are responsible for connecting two or more areas. An ABR contains the
full topological database for each area it is connected to and sends
this information to other areas. |
| Autonomous system boundary router (ASBR) | ASBRs connect to the outside world or perform some form of redistribution into OSPF. |
| Backbone router | Backbone
routers are connected to area 0, which is also represented as area
0.0.0.0. Backbone routers can be internal routers or ASBRs. |
Figure 4-1 displays a typical OSPF area assignment and the function of these routers.
In
Figure 4-1,
the routers residing in the backbone (area 0) are called backbone
routers. A backbone router connecting to another area can also be an
ABR. Routers that connect to, for example, the Internet and redistribute
external IP routing tables from such protocols as Border Gateway
Protocol (BGP) are termed autonomous system boundary routers (ASBRs).
So, you can have a backbone router perform ASBR functions as well as ABR
functions.
Each router, depending on its function, sends out a link-state
advertisement (LSA). An LSA is a packet used by such routing protocols
as OSPF (that is, link-state routing protocols) to send information to
neighboring routers describing networks and path costs.
OSPF supports a number of LSA types as well as three other area types: a
stub area, a totally stubby area, and a not-so-stubby area (NSSA).
These additional areas provide even more functionality in OSPF. Before
covering these new areas in detail, this section first goes over the
link-state advertisement types and when to use them in an OSPF
environment.
The OSPF standard defines a number of LSAs types. Unlike distance vector
protocols (for example, RIP), OSPF does not actually send its routing
table to other routers. Instead, OSPF sends the LSA database and derives
the IP routing table from LSAs. Table 4-2 describes the six most common
LSAs and their functions.
Table 4-2. Six Common Supported LSA Types on Cisco IOS Routers
| LSA Packet Type | Name | Function |
| 1 | Router link advertisements | Describes the state and cost of the router's own interfaces. |
| 2 | Network link advertisements | Used on multiaccess networks. These are originated by the designated router (DR). |
| 3 | Summary link advertisements (ABRs) | Originated
by ABRs only. This LSA type sends out information into the autonomous
system (AS) but outside of the area (interarea routes). |
| 4 | Summary link advertisements (ASBRs) | Originated by ASBRs describing IP networks external to the AS. |
| 5 | Autonomous system (AS) external link advertisements | An LSA sent to a router that connects to the Internet, for example. An advertisement sent from ABR to the ASBR. |
| 6 | Not-so-stubby areas (NSSA) | An advertisement bound to an NSSA area. |
A stub area is defined as an area that contains a single exit point from
the area. A stub in the English dictionary means a dead end, and that
is exactly what it means in OSPF. Areas that reside on the edge of the
network with no exit point except one path can be termed a stub area.
Stubs come in three types.
Table 4-3 summarizes the functions of these new areas, called stubby
areas, total stubby areas, and not-so-stubby areas. Take important note
of the LSA type allowed or not allowed to fully appreciate the value of a
stub area.
Table 4-3. Additional Area Types
| Area Type | Function |
| Stub area | This
area does not accept LSA types 4 and 5, which are summary links and
external link advertisements, respectively. The only way to achieve a
route to unknown destinations is, thereby, a default route injected by
the ABR. |
| Totally stubby area | This
area blocks LSA types 3, 4, and 5. Although similar to a stub area, a
totally stubby area blocks LSAs of type 3 as well. This solution is
Cisco-proprietary and is used to further reduce a topological database. |
| Not-so-stubby area | This
area is used primarily for connections to an ISP. This area is designed
to allow LSAs of type 7 only. All advertised routes can be flooded
through the NSSA but are blocked by the ABR. Basically, a type 7 LSA (if
the P bit is set to one) will be convert to a type 5 LSA and flooded
throughout the rest of the network. If the P bit is set to zero, no
translation takes place. Type 4 or 5 LSAs are not permitted. This
advertisement will not be propagated to the rest of the network.
Typically used to provide a default route. |
The only way to appreciate these new areas is to configure them and view
the OSPF database. The scenarios that follow cover stub, totally
stubby, and not-so-stubby areas in more detail.
Table 4-4 summarizes the LSA types by area and indicates which LSAs are permitted or disallowed in certain areas.
Table 4-4. LSA Types and Area Restrictions
| | LSA Type Permitted? |
| Area | 1/2 | 3/4 | 6 | 7 |
| NSSA | Yes | Yes | No | Yes |
| Totally stubby | Yes | No | No | No |
| Stub | Yes | Yes | No | No |
VLSM and Summarization with OSPF
OSPF supports a number of features. The two main features that interest
most network designers are that it supports VLSM and provides the
ability to summarize networks.
When an LSA packet or routing update is received or sent, the packet includes the following information:
- LSA type
- Router ID (unique IP address, no other router can share the same router ID)
- Subnet mask
- Attached router
- Metric
Because the subnet mask is carried along with the update, OSPF can
support VLSM. Without a mechanism that sends the subnet mask, there can
be no support for VLSM. Routing Information Protocol (RIPv1) and
Interior Gateway Routing Protocol (IGRP), for example, do not carry the
subnet mask when they send out updates.
Summarization occurs using the LSA type 4 packet or by the ASBR.
You configure OSPF in two ways to summarize networks using Cisco IOS routers:
- Interarea summarization creating type 3 or 4 LSAs
- External summarization with type 5 LSAs
Consider an OSPF network containing two routers across an Ethernet segment.
Figure 4-2 displays this two-router topology with the routers named R1 and R2.
R2 is sending R1 15 OSPF routes ranging from 131.109.1.0 to
131.109.15.0. Instead of populating R1's routing table with 15 IP route
entries, you can use summarization. Example 4-1 displays R1's routing
table.
Example 4-1. R1's OSPF Routing Table
R1>show ip route ospf
131.109.0.0/24 is subnetted, 14 subnets
O IA 131.109.14.0 [110/11] via 131.108.2.2, 00:00:48, Ethernet0/0
O IA 131.109.15.0 [110/11] via 131.108.2.2, 00:00:48, Ethernet0/0
O IA 131.109.12.0 [110/11] via 131.108.2.2, 00:00:48, Ethernet0/0
O IA 131.109.13.0 [110/11] via 131.108.2.2, 00:00:48, Ethernet0/0
O IA 131.109.10.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.11.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.8.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.9.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.6.0 [110/11] via 131.108.2.2, 00:00:00, Ethernet0/0
O IA 131.109.7.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.4.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.5.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.2.0 [110/11] via 131.108.2.2, 00:01:08, Ethernet0/0
O IA 131.109.3.0 [110/11] via 131.108.2.2, 00:00:58, Ethernet0/0
O IA 131.109.1.0 [110/11] via 131.108.2.2, 00:02:54, Ethernet0/0
The remote networks are indicated by O IA, which indicates interarea routes. Intra-area routes are indicated by O.
Example 4-1 displays an IP routing table telling you that R2 is in area 0
and another area (ABR); hence, R2 can perform interarea summarization.
Because the networks 1 to 15 are contiguous, you can configure R2 to
mask the networks by masking the first 15 networks with the IOS area area ID range address mask command. Example 4-2 displays the summary applied to R2 under the OSPF router process ID of 1.
Example 4-2. Summary of R2
R2(config)#router ospf 1
R2(config-router)#area 1 range 131.109.0.0 255.255.240.0
Example 4-3 displays R1's routing table now. Remember that previously there were 15 IP routing entries.
Example 4-3. OSPF Route Table on R1 After Summarization
R1#sh ip route ospf
131.109.0.0/20 is subnetted, 1 subnets
O IA 131.109.0.0 [110/11] via 131.108.2.2, 00:02:33, Ethernet0/0
R1#
By using OSPF summarization techniques, you can summarize a simple network with 15 IP networks by using 1 IP routing entry.
In OSPF, you can also externally summarize IP routes by using the summary ip-address mask command.
OSPF summarization examples are included among the five scenarios in this chapter.
OSPF over Multiarea NBMA
OSPF over a multiple-area NBMA network presents some challenges to a network designer as you discovered in Chapter 3.
Typically, in a large NBMA environment, the backbone (area 0) assignment
encompasses the NBMA connections themselves, because all remote or edge
sites need to transit the NBMA network. The same commands that applied
in Chapter 3 are used in large NBMA environments.
To summarize the command set used in large NBMA environments, the
following commands and steps are required to configure OSPF in a
multiarea OSPF Network:
- The network command enables OSPF across interfaces.
- Summarization enables networks to reduce IP routing table sizes by using area range on ABRs and the summary address subnet mask command for an ASBR.
- Any stubby configurations to reduce memory and CPU requirements.
- Any virtual links that may be required.
- Any command that manipulates the OSPF cost metrics for equal costs path load balancing.
Next, this chapter describes another common link-state routing protocol
used in large IP routing environments, namely Intermediate
System-to-Intermediate System (IS-IS).
