3. Splitting the AS into Areas
OSPF allows collections of contiguous networks and hosts to be
grouped together. Such a group, together with the routers having
interfaces to any one of the included networks, is called an area.
Each area runs a separate copy of the basic link-state routing
algorithm. This means that each area has its own link-state
database and corresponding graph, as explained in the previous
The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
detailed topology external to the area. This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic
as compared to treating the entire Autonomous System as a single
With the introduction of areas, it is no longer true that all
routers in the AS have an identical link-state database. A router
actually has a separate link-state database for each area it is
connected to. (Routers connected to multiple areas are called area
border routers). Two routers belonging to the same area have, for
that area, identical area link-state databases.
Routing in the Autonomous System takes place on two levels,
depending on whether the source and destination of a packet reside
in the same area (intra-area routing is used) or different areas
(inter-area routing is used). In intra-area routing, the packet is
routed solely on information obtained within the area; no routing
information obtained from outside the area can be used. This
protects intra-area routing from the injection of bad routing
information. We discuss inter-area routing in Section 3.2.
3.1. The backbone of the Autonomous System
The OSPF backbone is the special OSPF Area 0 (often written as
Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP
addresses). The OSPF backbone always contains all area border
routers. The backbone is responsible for distributing routing
information between non-backbone areas. The backbone must be
contiguous. However, it need not be physically contiguous;
backbone connectivity can be established/maintained through the
configuration of virtual links.
Virtual links can be configured between any two backbone routers
that have an interface to a common non-backbone area. Virtual
links belong to the backbone. The protocol treats two routers
joined by a virtual link as if they were connected by an
unnumbered point-to-point backbone network. On the graph of the
backbone, two such routers are joined by arcs whose costs are
the intra-area distances between the two routers. The routing
protocol traffic that flows along the virtual link uses intra-
area routing only.
3.2. Inter-area routing
When routing a packet between two non-backbone areas the
backbone is used. The path that the packet will travel can be
broken up into three contiguous pieces: an intra-area path from
the source to an area border router, a backbone path between the
source and destination areas, and then another intra-area path
to the destination. The algorithm finds the set of such paths
that have the smallest cost.
Looking at this another way, inter-area routing can be pictured
as forcing a star configuration on the Autonomous System, with
the backbone as hub and each of the non-backbone areas as
The topology of the backbone dictates the backbone paths used
between areas. The topology of the backbone can be enhanced by
adding virtual links. This gives the system administrator some
control over the routes taken by inter-area traffic.
The correct area border router to use as the packet exits the
source area is chosen in exactly the same way routers
advertising external routes are chosen. Each area border router
in an area summarizes for the area its cost to all networks
external to the area. After the SPF tree is calculated for the
area, routes to all inter-area destinations are calculated by
examining the summaries of the area border routers.
3.3. Classification of routers
Before the introduction of areas, the only OSPF routers having a
specialized function were those advertising external routing
information, such as Router RT5 in Figure 2. When the AS is
split into OSPF areas, the routers are further divided according
to function into the following four overlapping categories:
A router with all directly connected networks belonging to
the same area. These routers run a single copy of the basic
Area border routers
A router that attaches to multiple areas. Area border
routers run multiple copies of the basic algorithm, one copy
for each attached area. Area border routers condense the
topological information of their attached areas for
distribution to the backbone. The backbone in turn
distributes the information to the other areas.
A router that has an interface to the backbone area. This
includes all routers that interface to more than one area
(i.e., area border routers). However, backbone routers do
not have to be area border routers. Routers with all
interfaces connecting to the backbone area are supported.
AS boundary routers
A router that exchanges routing information with routers
belonging to other Autonomous Systems. Such a router
advertises AS external routing information throughout the
Autonomous System. The paths to each AS boundary router are
known by every router in the AS. This classification is
completely independent of the previous classifications: AS
boundary routers may be internal or area border routers, and
may or may not participate in the backbone.
3.4. A sample area configuration
Figure 6 shows a sample area configuration. The first area
consists of networks N1-N4, along with their attached routers
RT1-RT4. The second area consists of networks N6-N8, along with
their attached routers RT7, RT8, RT10 and RT11. The third area
consists of networks N9-N11 and Host H1, along with their
attached routers RT9, RT11 and RT12. The third area has been
configured so that networks N9-N11 and Host H1 will all be
grouped into a single route, when advertised external to the
area (see Section 3.5 for more details).
In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area
border routers. Finally, as before, Routers RT5 and RT7 are AS
Figure 7 shows the resulting link-state database for the Area 1.
The figure completely describes that area's intra-area routing.
It also shows the complete view of the internet for the two
internal routers RT1 and RT2. It is the job of the area border
routers, RT3 and RT4, to advertise into Area 1 the distances to
all destinations external to the area. These are indicated in
Figure 7 by the dashed stub routes. Also, RT3 and RT4 must
advertise into Area 1 the location of the AS boundary routers
RT5 and RT7. Finally, AS-external-LSAs from RT5 and RT7 are
flooded throughout the entire AS, and in particular throughout
Area 1. These LSAs are included in Area 1's database, and yield
routes to Networks N12-N15.
Routers RT3 and RT4 must also summarize Area 1's topology for
. N10 . . N7 .
. . .Area 2 .
.Area 3 . ................................
Figure 6: A sample OSPF area configuration
distribution to the backbone. Their backbone LSAs are shown in
Table 4. These summaries show which networks are contained in
Area 1 (i.e., Networks N1-N4), and the distance to these
networks from the routers RT3 and RT4 respectively.
The link-state database for the backbone is shown in Figure 8.
The set of routers pictured are the backbone routers. Router
RT11 is a backbone router because it belongs to two areas. In
order to make the backbone connected, a virtual link has been
configured between Routers R10 and R11.
The area border routers RT3, RT4, RT7, RT10 and RT11 condense
the routing information of their attached non-backbone areas for
distribution via the backbone; these are the dashed stubs that
appear in Figure 8. Remember that the third area has been
configured to condense Networks N9-N11 and Host H1 into a single
route. This yields a single dashed line for networks N9-N11 and
Host H1 in Figure 8. Routers RT5 and RT7 are AS boundary
routers; their externally derived information also appears on
the graph in Figure 8 as stubs.
Network RT3 adv. RT4 adv.
N1 4 4
N2 4 4
N3 1 1
N4 2 3
Table 4: Networks advertised to the backbone
by Routers RT3 and RT4.
Again using Routers RT3 and RT4 as an example, the procedure
goes as follows: They first calculate the SPF tree for the
backbone. This gives the distances to all other area border
routers. Also noted are the distances to networks (Ia and Ib)
and AS boundary routers (RT5 and RT7) that belong to the
backbone. This calculation is shown in Table 5.
Next, by looking at the area summaries from these area border
routers, RT3 and RT4 can determine the distance to all networks
outside their area. These distances are then advertised
internally to the area by RT3 and RT4. The advertisements that
Router RT3 and RT4 will make into Area 1 are shown in Table 6.
Note that Table 6 assumes that an area range has been configured
for the backbone which groups Ia and Ib into a single LSA.
The information imported into Area 1 by Routers RT3 and RT4
enables an internal router, such as RT1, to choose an area
border router intelligently. Router RT1 would use RT4 for
traffic to Network N6, RT3 for traffic to Network N10, and would
dist from dist from
to RT3 * 21
to RT4 22 *
to RT7 20 14
to RT10 15 22
to RT11 18 25
to Ia 20 27
to Ib 15 22
to RT5 14 8
to RT7 20 14
Table 5: Backbone distances calculated
by Routers RT3 and RT4.
Destination RT3 adv. RT4 adv.
Ia,Ib 20 27
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 29 36
RT5 14 8
RT7 20 14
Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4.
load share between the two for traffic to Network N8.
Router RT1 can also determine in this manner the shortest path
to the AS boundary routers RT5 and RT7. Then, by looking at RT5
and RT7's AS-external-LSAs, Router RT1 can decide between RT5 or
RT7 when sending to a destination in another Autonomous System
(one of the networks N12-N15).
Note that a failure of the line between Routers RT6 and RT10
will cause the backbone to become disconnected. Configuring a
virtual link between Routers RT7 and RT10 will give the backbone
more connectivity and more resistance to such failures.
3.5. IP subnetting support
OSPF attaches an IP address mask to each advertised route. The
mask indicates the range of addresses being described by the
particular route. For example, a summary-LSA for the
destination 18.104.22.168 with a mask of 0xffff0000 actually is
describing a single route to the collection of destinations
22.214.171.124 - 126.96.36.199. Similarly, host routes are
always advertised with a mask of 0xffffffff, indicating the
presence of only a single destination.
Including the mask with each advertised destination enables the
implementation of what is commonly referred to as variable-
length subnetting. This means that a single IP class A, B, or C
network number can be broken up into many subnets of various
sizes. For example, the network 188.8.131.52 could be broken up
into 62 variable-sized subnets: 15 subnets of size 4K, 15
subnets of size 256, and 32 subnets of size 8. Table 7 shows
some of the resulting network addresses together with their
Network address IP address mask Subnet size
184.108.40.206 0xfffff000 4K
220.127.116.11 0xffffff00 256
18.104.22.168 0xfffffff8 8
Table 7: Some sample subnet sizes.
There are many possible ways of dividing up a class A, B, and C
network into variable sized subnets. The precise procedure for
doing so is beyond the scope of this specification. This
specification however establishes the following guideline: When
an IP packet is forwarded, it is always forwarded to the network
that is the best match for the packet's destination. Here best
match is synonymous with the longest or most specific match.
For example, the default route with destination of 0.0.0.0 and
mask 0x00000000 is always a match for every IP destination. Yet
it is always less specific than any other match. Subnet masks
must be assigned so that the best match for any IP destination
Attaching an address mask to each route also enables the support
of IP supernetting. For example, a single physical network
segment could be assigned the [address,mask] pair
[22.214.171.124,0xfffffc00]. The segment would then be single IP
network, containing addresses from the four consecutive class C
network numbers 126.96.36.199 through 188.8.131.52. Such addressing is
now becoming commonplace with the advent of CIDR (see [Ref10]).
In order to get better aggregation at area boundaries, area
address ranges can be employed (see Section C.2 for more
details). Each address range is defined as an [address,mask]
pair. Many separate networks may then be contained in a single
address range, just as a subnetted network is composed of many
separate subnets. Area border routers then summarize the area
contents (for distribution to the backbone) by advertising a
single route for each address range. The cost of the route is
the maximum cost to any of the networks falling in the specified
For example, an IP subnetted network might be configured as a
single OSPF area. In that case, a single address range could be
configured: a class A, B, or C network number along with its
natural IP mask. Inside the area, any number of variable sized
subnets could be defined. However, external to the area a
single route for the entire subnetted network would be
distributed, hiding even the fact that the network is subnetted
at all. The cost of this route is the maximum of the set of
costs to the component subnets.
3.6. Supporting stub areas
In some Autonomous Systems, the majority of the link-state
database may consist of AS-external-LSAs. An OSPF AS-external-
LSA is usually flooded throughout the entire AS. However, OSPF
allows certain areas to be configured as "stub areas". AS-
external-LSAs are not flooded into/throughout stub areas;
routing to AS external destinations in these areas is based on a
(per-area) default only. This reduces the link-state database
size, and therefore the memory requirements, for a stub area's
In order to take advantage of the OSPF stub area support,
default routing must be used in the stub area. This is
accomplished as follows. One or more of the stub area's area
border routers must advertise a default route into the stub area
via summary-LSAs. These summary defaults are flooded throughout
the stub area, but no further. (For this reason these defaults
pertain only to the particular stub area). These summary
default routes will be used for any destination that is not
explicitly reachable by an intra-area or inter-area path (i.e.,
AS external destinations).
An area can be configured as a stub when there is a single exit
point from the area, or when the choice of exit point need not
be made on a per-external-destination basis. For example, Area
3 in Figure 6 could be configured as a stub area, because all
external traffic must travel though its single area border
router RT11. If Area 3 were configured as a stub, Router RT11
would advertise a default route for distribution inside Area 3
(in a summary-LSA), instead of flooding the AS-external-LSAs for
Networks N12-N15 into/throughout the area.
The OSPF protocol ensures that all routers belonging to an area
agree on whether the area has been configured as a stub. This
guarantees that no confusion will arise in the flooding of AS-
There are a couple of restrictions on the use of stub areas.
Virtual links cannot be configured through stub areas. In
addition, AS boundary routers cannot be placed internal to stub
3.7. Partitions of areas
OSPF does not actively attempt to repair area partitions. When
an area becomes partitioned, each component simply becomes a
separate area. The backbone then performs routing between the
new areas. Some destinations reachable via intra-area routing
before the partition will now require inter-area routing.
However, in order to maintain full routing after the partition,
an address range must not be split across multiple components of
the area partition. Also, the backbone itself must not
partition. If it does, parts of the Autonomous System will
become unreachable. Backbone partitions can be repaired by
configuring virtual links (see Section 15).
Another way to think about area partitions is to look at the
Autonomous System graph that was introduced in Section 2. Area
IDs can be viewed as colors for the graph's edges. Each edge
of the graph connects to a network, or is itself a point-to-
point network. In either case, the edge is colored with the
network's Area ID.
A group of edges, all having the same color, and interconnected
by vertices, represents an area. If the topology of the
Autonomous System is intact, the graph will have several regions
of color, each color being a distinct Area ID.
When the AS topology changes, one of the areas may become
partitioned. The graph of the AS will then have multiple
regions of the same color (Area ID). The routing in the
Autonomous System will continue to function as long as these
regions of same color are connected by the single backbone
4. Functional Summary
A separate copy of OSPF's basic routing algorithm runs in each area.
Routers having interfaces to multiple areas run multiple copies of
the algorithm. A brief summary of the routing algorithm follows.
When a router starts, it first initializes the routing protocol data
structures. The router then waits for indications from the lower-
level protocols that its interfaces are functional.
A router then uses the OSPF's Hello Protocol to acquire neighbors.
The router sends Hello packets to its neighbors, and in turn
receives their Hello packets. On broadcast and point-to-point
networks, the router dynamically detects its neighboring routers by
sending its Hello packets to the multicast address AllSPFRouters.
On non-broadcast networks, some configuration information may be
necessary in order to discover neighbors. On broadcast and NBMA
networks the Hello Protocol also elects a Designated router for the
The router will attempt to form adjacencies with some of its newly
acquired neighbors. Link-state databases are synchronized between
pairs of adjacent routers. On broadcast and NBMA networks, the
Designated Router determines which routers should become adjacent.
Adjacencies control the distribution of routing information.
Routing updates are sent and received only on adjacencies.
A router periodically advertises its state, which is also called
link state. Link state is also advertised when a router's state
changes. A router's adjacencies are reflected in the contents of
its LSAs. This relationship between adjacencies and link state
allows the protocol to detect dead routers in a timely fashion.
LSAs are flooded throughout the area. The flooding algorithm is
reliable, ensuring that all routers in an area have exactly the same
link-state database. This database consists of the collection of
LSAs originated by each router belonging to the area. From this
database each router calculates a shortest-path tree, with itself as
root. This shortest-path tree in turn yields a routing table for
4.1. Inter-area routing
The previous section described the operation of the protocol
within a single area. For intra-area routing, no other routing
information is pertinent. In order to be able to route to
destinations outside of the area, the area border routers inject
additional routing information into the area. This additional
information is a distillation of the rest of the Autonomous
This distillation is accomplished as follows: Each area border
router is by definition connected to the backbone. Each area
border router summarizes the topology of its attached non-
backbone areas for transmission on the backbone, and hence to
all other area border routers. An area border router then has
complete topological information concerning the backbone, and
the area summaries from each of the other area border routers.
From this information, the router calculates paths to all
inter-area destinations. The router then advertises these paths
into its attached areas. This enables the area's internal
routers to pick the best exit router when forwarding traffic
4.2. AS external routes
Routers that have information regarding other Autonomous Systems
can flood this information throughout the AS. This external
routing information is distributed verbatim to every
participating router. There is one exception: external routing
information is not flooded into "stub" areas (see Section 3.6).
To utilize external routing information, the path to all routers
advertising external information must be known throughout the AS
(excepting the stub areas). For that reason, the locations of
these AS boundary routers are summarized by the (non-stub) area
4.3. Routing protocol packets
The OSPF protocol runs directly over IP, using IP protocol 89.
OSPF does not provide any explicit fragmentation/reassembly
support. When fragmentation is necessary, IP
fragmentation/reassembly is used. OSPF protocol packets have
been designed so that large protocol packets can generally be
split into several smaller protocol packets. This practice is
recommended; IP fragmentation should be avoided whenever
Routing protocol packets should always be sent with the IP TOS
field set to 0. If at all possible, routing protocol packets
should be given preference over regular IP data traffic, both
when being sent and received. As an aid to accomplishing this,
OSPF protocol packets should have their IP precedence field set
to the value Internetwork Control (see [Ref5]).
All OSPF protocol packets share a common protocol header that is
described in Appendix A. The OSPF packet types are listed below
in Table 8. Their formats are also described in Appendix A.
Type Packet name Protocol function
1 Hello Discover/maintain neighbors
2 Database Description Summarize database contents
3 Link State Request Database download
4 Link State Update Database update
5 Link State Ack Flooding acknowledgment
Table 8: OSPF packet types.
OSPF's Hello protocol uses Hello packets to discover and
maintain neighbor relationships. The Database Description and
Link State Request packets are used in the forming of
adjacencies. OSPF's reliable update mechanism is implemented by
the Link State Update and Link State Acknowledgment packets.
Each Link State Update packet carries a set of new link state
advertisements (LSAs) one hop further away from their point of
origination. A single Link State Update packet may contain the
LSAs of several routers. Each LSA is tagged with the ID of the
originating router and a checksum of its link state contents.
Each LSA also has a type field; the different types of OSPF LSAs
are listed below in Table 9.
OSPF routing packets (with the exception of Hellos) are sent
only over adjacencies. This means that all OSPF protocol
packets travel a single IP hop, except those that are sent over
virtual adjacencies. The IP source address of an OSPF protocol
packet is one end of a router adjacency, and the IP destination
address is either the other end of the adjacency or an IP
4.4. Basic implementation requirements
An implementation of OSPF requires the following pieces of
Two different kind of timers are required. The first kind,
called "single shot timers", fire once and cause a protocol
event to be processed. The second kind, called "interval
timers", fire at continuous intervals. These are used for
the sending of packets at regular intervals. A good example
of this is the regular broadcast of Hello packets. The
granularity of both kinds of timers is one second.
Interval timers should be implemented to avoid drift. In
some router implementations, packet processing can affect
timer execution. When multiple routers are attached to a
single network, all doing broadcasts, this can lead to the
synchronization of routing packets (which should be
avoided). If timers cannot be implemented to avoid drift,
small random amounts should be added to/subtracted from the
interval timer at each firing.
LS LSA LSA description
1 Router-LSAs Originated by all routers.
This LSA describes
the collected states of the
router's interfaces to an
area. Flooded throughout a
single area only.
2 Network-LSAs Originated for broadcast
and NBMA networks by
the Designated Router. This
LSA contains the
list of routers connected
to the network. Flooded
throughout a single area only.
3,4 Summary-LSAs Originated by area border
routers, and flooded through-
out the LSA's associated
area. Each summary-LSA
describes a route to a
destination outside the area,
yet still inside the AS
(i.e., an inter-area route).
Type 3 summary-LSAs describe
routes to networks. Type 4
routes to AS boundary routers.
5 AS-external-LSAs Originated by AS boundary
routers, and flooded through-
out the AS. Each
a route to a destination in
another Autonomous System.
Default routes for the AS can
also be described by
Table 9: OSPF link state advertisements (LSAs).
Certain OSPF packets take the form of IP multicast
datagrams. Support for receiving and sending IP multicast
datagrams, along with the appropriate lower-level protocol
support, is required. The IP multicast datagrams used by
OSPF never travel more than one hop. For this reason, the
ability to forward IP multicast datagrams is not required.
For information on IP multicast, see [Ref7].
Variable-length subnet support
The router's IP protocol support must include the ability to
divide a single IP class A, B, or C network number into many
subnets of various sizes. This is commonly called
variable-length subnetting; see Section 3.5 for details.
IP supernetting support
The router's IP protocol support must include the ability to
aggregate contiguous collections of IP class A, B, and C
networks into larger quantities called supernets.
Supernetting has been proposed as one way to improve the
scaling of IP routing in the worldwide Internet. For more
information on IP supernetting, see [Ref10].
Lower-level protocol support
The lower level protocols referred to here are the network
access protocols, such as the Ethernet data link layer.
Indications must be passed from these protocols to OSPF as
the network interface goes up and down. For example, on an
ethernet it would be valuable to know when the ethernet
transceiver cable becomes unplugged.
Non-broadcast lower-level protocol support
On non-broadcast networks, the OSPF Hello Protocol can be
aided by providing an indication when an attempt is made to
send a packet to a dead or non-existent router. For
example, on an X.25 PDN a dead neighboring router may be
indicated by the reception of a X.25 clear with an
appropriate cause and diagnostic, and this information would
be passed to OSPF.
List manipulation primitives
Much of the OSPF functionality is described in terms of its
operation on lists of LSAs. For example, the collection of
LSAs that will be retransmitted to an adjacent router until
acknowledged are described as a list. Any particular LSA
may be on many such lists. An OSPF implementation needs to
be able to manipulate these lists, adding and deleting
constituent LSAs as necessary.
Certain procedures described in this specification invoke
other procedures. At times, these other procedures should
be executed in-line, that is, before the current procedure
is finished. This is indicated in the text by instructions
to execute a procedure. At other times, the other
procedures are to be executed only when the current
procedure has finished. This is indicated by instructions
to schedule a task.
4.5. Optional OSPF capabilities
The OSPF protocol defines several optional capabilities. A
router indicates the optional capabilities that it supports in
its OSPF Hello packets, Database Description packets and in its
LSAs. This enables routers supporting a mix of optional
capabilities to coexist in a single Autonomous System.
Some capabilities must be supported by all routers attached to a
specific area. In this case, a router will not accept a
neighbor's Hello Packet unless there is a match in reported
capabilities (i.e., a capability mismatch prevents a neighbor
relationship from forming). An example of this is the
ExternalRoutingCapability (see below).
Other capabilities can be negotiated during the Database
Exchange process. This is accomplished by specifying the
optional capabilities in Database Description packets. A
capability mismatch with a neighbor in this case will result in
only a subset of the link state database being exchanged between
the two neighbors.
The routing table build process can also be affected by the
presence/absence of optional capabilities. For example, since
the optional capabilities are reported in LSAs, routers
incapable of certain functions can be avoided when building the
shortest path tree.
The OSPF optional capabilities defined in this memo are listed
below. See Section A.2 for more information.
Entire OSPF areas can be configured as "stubs" (see Section
3.6). AS-external-LSAs will not be flooded into stub areas.
This capability is represented by the E-bit in the OSPF
Options field (see Section A.2). In order to ensure
consistent configuration of stub areas, all routers
interfacing to such an area must have the E-bit clear in
their Hello packets (see Sections 9.5 and 10.5).
5. Protocol Data Structures
The OSPF protocol is described herein in terms of its operation on
various protocol data structures. The following list comprises the
top-level OSPF data structures. Any initialization that needs to be
done is noted. OSPF areas, interfaces and neighbors also have
associated data structures that are described later in this
A 32-bit number that uniquely identifies this router in the AS.
One possible implementation strategy would be to use the
smallest IP interface address belonging to the router. If a
router's OSPF Router ID is changed, the router's OSPF software
should be restarted before the new Router ID takes effect. In
this case the router should flush its self-originated LSAs from
the routing domain (see Section 14.1) before restarting, or they
will persist for up to MaxAge minutes.
Each one of the areas to which the router is connected has its
own data structure. This data structure describes the working
of the basic OSPF algorithm. Remember that each area runs a
separate copy of the basic OSPF algorithm.
Backbone (area) structure
The OSPF backbone area is responsible for the dissemination of
inter-area routing information.
Virtual links configured
The virtual links configured with this router as one endpoint.
In order to have configured virtual links, the router itself
must be an area border router. Virtual links are identified by
the Router ID of the other endpoint -- which is another area
border router. These two endpoint routers must be attached to a
common area, called the virtual link's Transit area. Virtual
links are part of the backbone, and behave as if they were
unnumbered point-to-point networks between the two routers. A
virtual link uses the intra-area routing of its Transit area to
forward packets. Virtual links are brought up and down through
the building of the shortest-path trees for the Transit area.
List of external routes
These are routes to destinations external to the Autonomous
System, that have been gained either through direct experience
with another routing protocol (such as BGP), or through
configuration information, or through a combination of the two
(e.g., dynamic external information to be advertised by OSPF
with configured metric). Any router having these external routes
is called an AS boundary router. These routes are advertised by
the router into the OSPF routing domain via AS-external-LSAs.
List of AS-external-LSAs
Part of the link-state database. These have originated from the
AS boundary routers. They comprise routes to destinations
external to the Autonomous System. Note that, if the router is
itself an AS boundary router, some of these AS-external-LSAs
have been self-originated.
The routing table
Derived from the link-state database. Each entry in the routing
table is indexed by a destination, and contains the
destination's cost and a set of paths to use in forwarding
packets to the destination. A path is described by its type and
next hop. For more information, see Section 11.
Figure 9 shows the collection of data structures present in a
typical router. The router pictured is RT10, from the map in Figure
6. Note that Router RT10 has a virtual link configured to Router
RT11, with Area 2 as the link's Transit area. This is indicated by
the dashed line in Figure 9. When the virtual link becomes active,
through the building of the shortest path tree for Area 2, it
becomes an interface to the backbone (see the two backbone
interfaces depicted in Figure 9).
6. The Area Data Structure
The area data structure contains all the information used to run the
basic OSPF routing algorithm. Each area maintains its own link-state
database. A network belongs to a single area, and a router interface
connects to a single area. Each router adjacency also belongs to a
The OSPF backbone is the special OSPF area responsible for
disseminating inter-area routing information.
The area link-state database consists of the collection of router-
LSAs, network-LSAs and summary-LSAs that have originated from the
area's routers. This information is flooded throughout a single
area only. The list of AS-external-LSAs (see Section 5) is also
considered to be part of each area's link-state database.
A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
reserved for the backbone.
List of area address ranges
In order to aggregate routing information at area boundaries,
area address ranges can be employed. Each address range is
specified by an [address,mask] pair and a status indication of
either Advertise or DoNotAdvertise (see Section 12.4.3).
/ \ |Routing Table|
/ \ +-------------+
+------+ / \ +--------+
|Area 2|---+ +---|Backbone|
/ \ * / \
/ \ * / \
+---------+ +---------+ +------------+ +------------+
|Interface| |Interface| |Virtual Link| |Interface Ib|
| to N6 | | to N8 | | to RT11 | +------------+
+---------+ +---------+ +------------+ |
/ \ | | |
/ \ | | |
+--------+ +--------+ | +-------------+ +------------+
|Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6|
| RT8 | | RT7 | | +-------------+ +------------+
+--------+ +--------+ |
Figure 9: Router RT10's Data structures
Associated router interfaces
This router's interfaces connecting to the area. A router
interface belongs to one and only one area (or the backbone).
For the backbone area this list includes all the virtual links.
A virtual link is identified by the Router ID of its other
endpoint; its cost is the cost of the shortest intra-area path
through the Transit area that exists between the two routers.
List of router-LSAs
A router-LSA is generated by each router in the area. It
describes the state of the router's interfaces to the area.
List of network-LSAs
One network-LSA is generated for each transit broadcast and NBMA
network in the area. A network-LSA describes the set of routers
currently connected to the network.
List of summary-LSAs
Summary-LSAs originate from the area's area border routers.
They describe routes to destinations internal to the Autonomous
System, yet external to the area (i.e., inter-area
The shortest-path tree for the area, with this router itself as
root. Derived from the collected router-LSAs and network-LSAs
by the Dijkstra algorithm (see Section 16.1).
This parameter indicates whether the area can carry data traffic
that neither originates nor terminates in the area itself. This
parameter is calculated when the area's shortest-path tree is
built (see Section 16.1, where TransitCapability is set to TRUE
if and only if there are one or more fully adjacent virtual
links using the area as Transit area), and is used as an input
to a subsequent step of the routing table build process (see
Section 16.3). When an area's TransitCapability is set to TRUE,
the area is said to be a "transit area".
Whether AS-external-LSAs will be flooded into/throughout the
area. This is a configurable parameter. If AS-external-LSAs
are excluded from the area, the area is called a "stub". Within
stub areas, routing to AS external destinations will be based
solely on a default summary route. The backbone cannot be
configured as a stub area. Also, virtual links cannot be
configured through stub areas. For more information, see
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost
indicates the cost of the default summary-LSA that the router
should advertise into the area. See Section 12.4.3 for more
Unless otherwise specified, the remaining sections of this document
refer to the operation of the OSPF protocol within a single area.