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 topological
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 topological database. A router
actually has a separate topological 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 topological 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 backbone consists of those networks not contained in any
area, their attached routers, and those routers that belong to
multiple areas. The backbone must be contiguous.
It is possible to define areas in such a way that the backbone
is no longer contiguous. In this case the system administrator
must restore backbone connectivity by configuring 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 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.
The backbone is responsible for distributing routing information
between areas. The backbone itself has all of the properties of
an area. The topology of the backbone is invisible to each of
the areas, while the backbone itself knows nothing of the
topology of the areas.
3.2. Inter-area routing
When routing a packet between two 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 areas as spokes.
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 other networks 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. Routers with only backbone interfaces also
belong to this category. These routers run a single copy of
the basic routing algorithm.
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 and an additional copy for the
backbone. 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. 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 connected to the backbone are considered to be
AS boundary routers
A router that exchanges routing information with routers
belonging to other Autonomous Systems. Such a router has AS
external routes that are advertised throughout the
Autonomous System. The path to each AS boundary router is
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 topological 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, external advertisements from RT5
and RT7 are flooded throughout the entire AS, and in particular
throughout Area 1. These advertisements 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
distribution to the backbone. Their backbone advertisements 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.
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.
The topological 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.
Again, Routers RT3, RT4, RT7, RT10 and RT11 are area border
routers. As Routers RT3 and RT4 did above, they have condensed
the routing information of their attached 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.
The backbone enables the exchange of summary information between
area border routers. Every area border router hears the area
summaries from all other area border routers. It then forms a
picture of the distance to all networks outside of its area by
examining the collected advertisements, and adding in the
backbone distance to each advertising router.
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.
Area border dist from dist from
router RT3 RT4
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.
Note that Table 6 assumes that an area range has been configured
for the backbone which groups Ia and Ib into a single
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
load share between the two for traffic to Network N8.
Destination RT3 adv. RT4 adv.
Ia,Ib 15 22
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 19 26
RT5 14 8
RT7 20 14
Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4.
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 external advertisements, 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. Also, a
virtual link between RT7 and RT10 would allow a much shorter
path between the third area (containing N9) and the router RT7,
which is advertising a good route to external network N12.
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 advertisement for the
destination 126.96.36.199 with a mask of 0xffff0000 actually is
describing a single route to the collection of destinations
188.8.131.52 - 184.108.40.206. 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 220.127.116.11 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
18.104.22.168 0xfffff000 4K
22.214.171.124 0xffffff00 256
126.96.36.199 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
The OSPF area concept is modelled after an IP subnetted network.
OSPF areas have been loosely defined to be a collection of
networks. In actuality, an OSPF area is specified to be a list
of address ranges (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 minimum
cost to any of the networks falling in the specified range.
For example, an IP subnetted network can be configured as a
single OSPF area. In that case, the area would be defined as a
single address range: 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. 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 minimum of the set of
costs to the component subnets.
3.6. Supporting stub areas
In some Autonomous Systems, the majority of the topological
database may consist of AS external advertisements. An OSPF AS
external advertisement is usually flooded throughout the entire
AS. However, OSPF allows certain areas to be configured as
"stub areas". AS external advertisements 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 topological database size, and therefore the memory
requirements, for a stub area's internal routers.
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 link advertisements. 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 match 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 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 link advertisement), instead of flooding the AS
external advertisements for Networks N12-N15 into/throughout the
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.
In the previous section, an area was described as a list of
address ranges. Any particular address range must still be
completely contained in a single component of the area
partition. This has to do with the way the area contents are
summarized to the backbone. 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 is
necessary in order to discover neighbors. On all multi-access
networks (broadcast or non-broadcast), the Hello Protocol also
elects a Designated router for the network.
The router will attempt to form adjacencies with some of its newly
acquired neighbors. Topological databases are synchronized between
pairs of adjacent routers. On multi-access networks, the Designated
Router determines which routers should become adjacent.
Adjacencies control the distribution of routing protocol packets.
Routing protocol packets are sent and received only on adjacencies.
In particular, distribution of topological database updates proceeds
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 link state advertisements. This relationship between
adjacencies and link state allows the protocol to detect dead
routers in a timely fashion.
Link state advertisements are flooded throughout the area. The
flooding algorithm is reliable, ensuring that all routers in an area
have exactly the same topological database. This database consists
of the collection of link state advertisements received from 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 the protocol.
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 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 destinations not contained in
its attached areas. 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 to
destinations in other areas.
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 [RFC 791]).
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 one hop further away from their point of
origination. A single Link State Update packet may contain the
link state advertisements of several routers. Each
advertisement is tagged with the ID of the originating router
and a checksum of its link state contents. The five different
types of OSPF link state advertisements are listed below in
As mentioned above, OSPF routing packets (with the exception of
Hellos) are sent only over adjacencies. Note that 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
LS Advertisement Advertisement description
1 Router links Originated by all routers.
advertisements This advertisement describes
the collected states of the
router's interfaces to an
area. Flooded throughout a
single area only.
2 Network links Originated for multi-access
advertisements networks by the Designated
Router. This advertisement
contains the list of routers
connected to the network.
Flooded throughout a single
3,4 Summary link Originated by area border
advertisements routers, and flooded through-
out the advertisement's
associated area. Each summary
link advertisement describes
a route to a destination out-
side the area, yet still inside
the AS (i.e., an inter-area
route). Type 3 advertisements
describe routes to networks.
Type 4 advertisements describe
routes to AS boundary routers.
5 AS external link Originated by AS boundary
advertisements routers, and flooded through-
out the AS. Each AS external
link advertisement describes
a route to a destination in
another Autonomous System.
Default routes for the AS can
also be described by AS
external link advertisements.
Table 9: OSPF link state advertisements.
end of the adjacency or an IP multicast address.
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 (on
broadcast networks). 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
timer interval at each firing.
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 [RFC 1112].
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 [RFC 1519].
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
Remember that non-broadcast networks are multi-access
networks such as a X.25 PDN. On these networks, the Hello
Protocol can be aided by providing an indication to OSPF
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 link state advertisements. For
example, the collection of advertisements that will be
retransmitted to an adjacent router until acknowledged are
described as a list. Any particular advertisement may be on
many such lists. An OSPF implementation needs to be able to
manipulate these lists, adding and deleting constituent
advertisements 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
link state advertisements. This enables routers supporting a
mix of optional capabilities to coexist in a single Autonomous
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 link state advertisements 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 link state
advertisements, routers incapable of certain functions can be
avoided when building the shortest path tree. An example of
this is the TOS routing capability (see below).
The current OSPF optional capabilities are listed below. See
Section A.2 for more information.
Entire OSPF areas can be configured as "stubs" (see Section
3.6). AS external advertisements 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).
All OSPF implementations must be able to calculate separate
routes based on IP Type of Service. However, to save
routing table space and processing resources, an OSPF router
can be configured to ignore TOS when forwarding packets. In
this case, the router calculates routes for TOS 0 only.
This capability is represented by the T-bit in the OSPF
options field (see Section A.2). TOS-capable routers will
attempt to avoid non-TOS-capable routers when calculating
non-zero TOS paths.
5. Protocol Data Structures
The OSPF protocol is described in this specification 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 specification.
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.
Before restarting in order to change its Router ID, the router
should flush its self-originated link state advertisements from
the routing domain (see Section 14.1), 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 algorithm. Remember that each area runs a separate
copy of the basic algorithm.
Backbone (area) structure
The basic algorithm operates on the backbone as if it were an
area. For this reason the backbone is represented as an area
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 EGP), 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 link
List of AS external link advertisements
Part of the topological 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 link
advertisements have been self-originated.
The routing table
Derived from the topological database. Each destination that
the router can forward to is represented by a cost and a set of
paths. A path is described by its type and next hop. For more
information, see Section 11.
This item indicates whether the router will calculate separate
routes based on TOS. This is a configurable parameter. For
more information, see Sections 4.5 and 16.9.
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 routing algorithm. Each area maintains its own topological
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 has all the properties of an area. For that
reason it is also represented by an area data structure. Note that
some items in the structure apply differently to the backbone than
to non-backbone areas.
/ \ |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
The area topological (or link state) database consists of the
collection of router links, network links and summary link
advertisements that have originated from the area's routers. This
information is flooded throughout a single area only. The list of
AS external link advertisements (see Section 5) is also considered
to be part of each area's topological database.
A 32-bit number identifying the area. 0.0.0.0 is reserved for
the Area ID of the backbone. If assigning subnetted networks as
separate areas, the IP network number could be used as the Area
List of component address ranges
The address ranges that define the area. Each address range is
specified by an [address,mask] pair and a status indication of
either Advertise or DoNotAdvertise (see Section 12.4.3). Each
network is then assigned to an area depending on the address
range that it falls into (specified address ranges are not
allowed to overlap). As an example, if an IP subnetted network
is to be its own separate OSPF area, the area is defined to
consist of a single address range - an IP network number with
its natural (class A, B or C) mask.
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 structure 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
List of router links advertisements
A router links advertisement is generated by each router in the
area. It describes the state of the router's interfaces to the
List of network links advertisements
One network links advertisement is generated for each transit
multi-access network in the area. A network links advertisement
describes the set of routers currently connected to the network.
List of summary link advertisements
Summary link advertisements originate from the area's area
border routers. They describe routes to destinations internal
to the Autonomous System, yet external to the area.
The shortest-path tree for the area, with this router itself as
root. Derived from the collected router links and network links
advertisements by the Dijkstra algorithm (see Section 16.1).
The type of authentication used for this area. Authentication
types are defined in Appendix D. All OSPF packet exchanges are
authenticated. Different authentication schemes may be used in
Set to TRUE if and only if there are one or more active virtual
links using the area as a Transit area. Equivalently, 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, and is used as an input to a subsequent
step of the routing table build process (see Section 16.3).
Whether AS external advertisements will be flooded
into/throughout the area. This is a configurable parameter. If
AS external advertisements are excluded from the area, the area
is called a "stub". Internal to 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 Section 3.6.
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 link that the router
should advertise into the area. There can be a separate cost
configured for each IP TOS. See Section 12.4.3 for more
Unless otherwise specified, the remaining sections of this document
refer to the operation of the protocol in a single area.
7. Bringing Up Adjacencies
OSPF creates adjacencies between neighboring routers for the purpose
of exchanging routing information. Not every two neighboring
routers will become adjacent. This section covers the generalities
involved in creating adjacencies. For further details consult
7.1. The Hello Protocol
The Hello Protocol is responsible for establishing and
maintaining neighbor relationships. It also ensures that
communication between neighbors is bidirectional. Hello packets
are sent periodically out all router interfaces. Bidirectional
communication is indicated when the router sees itself listed in
the neighbor's Hello Packet.
On multi-access networks, the Hello Protocol elects a Designated
Router for the network. Among other things, the Designated
Router controls what adjacencies will be formed over the network
The Hello Protocol works differently on broadcast networks, as
compared to non-broadcast networks. On broadcast networks, each
router advertises itself by periodically multicasting Hello
Packets. This allows neighbors to be discovered dynamically.
These Hello Packets contain the router's view of the Designated
Router's identity, and the list of routers whose Hello Packets
have been seen recently.
On non-broadcast networks some configuration information is
necessary for the operation of the Hello Protocol. Each router
that may potentially become Designated Router has a list of all
other routers attached to the network. A router, having
Designated Router potential, sends Hello Packets to all other
potential Designated Routers when its interface to the non-
broadcast network first becomes operational. This is an attempt
to find the Designated Router for the network. If the router
itself is elected Designated Router, it begins sending Hello
Packets to all other routers attached to the network.
After a neighbor has been discovered, bidirectional
communication ensured, and (if on a multi-access network) a
Designated Router elected, a decision is made regarding whether
or not an adjacency should be formed with the neighbor (see
Section 10.4). An attempt is always made to establish
adjacencies over point-to-point networks and virtual links. The
first step in bringing up an adjacency is to synchronize the
neighbors' topological databases. This is covered in the next
7.2. The Synchronization of Databases
In a link-state routing algorithm, it is very important for all
routers' topological databases to stay synchronized. OSPF
simplifies this by requiring only adjacent routers to remain
synchronized. The synchronization process begins as soon as the
routers attempt to bring up the adjacency. Each router
describes its database by sending a sequence of Database
Description packets to its neighbor. Each Database Description
Packet describes a set of link state advertisements belonging to
the router's database. When the neighbor sees a link state
advertisement that is more recent than its own database copy, it
makes a note that this newer advertisement should be requested.
This sending and receiving of Database Description packets is
called the "Database Exchange Process". During this process,
the two routers form a master/slave relationship. Each Database
Description Packet has a sequence number. Database Description
Packets sent by the master (polls) are acknowledged by the slave
through echoing of the sequence number. Both polls and their
responses contain summaries of link state data. The master is
the only one allowed to retransmit Database Description Packets.
It does so only at fixed intervals, the length of which is the
configured constant RxmtInterval.
Each Database Description contains an indication that there are
more packets to follow --- the M-bit. The Database Exchange
Process is over when a router has received and sent Database
Description Packets with the M-bit off.
During and after the Database Exchange Process, each router has
a list of those link state advertisements for which the neighbor
has more up-to-date instances. These advertisements are
requested in Link State Request Packets. Link State Request
packets that are not satisfied are retransmitted at fixed
intervals of time RxmtInterval. When the Database Description
Process has completed and all Link State Requests have been
satisfied, the databases are deemed synchronized and the routers
are marked fully adjacent. At this time the adjacency is fully
functional and is advertised in the two routers' link state
The adjacency is used by the flooding procedure as soon as the
Database Exchange Process begins. This simplifies database
synchronization, and guarantees that it finishes in a
predictable period of time.
7.3. The Designated Router
Every multi-access network has a Designated Router. The
Designated Router performs two main functions for the routing
o The Designated Router originates a network links
advertisement on behalf of the network. This advertisement
lists the set of routers (including the Designated Router
itself) currently attached to the network. The Link State
ID for this advertisement (see Section 12.1.4) is the IP
interface address of the Designated Router. The IP network
number can then be obtained by using the subnet/network
o The Designated Router becomes adjacent to all other routers
on the network. Since the link state databases are
synchronized across adjacencies (through adjacency bring-up
and then the flooding procedure), the Designated Router
plays a central part in the synchronization process.
The Designated Router is elected by the Hello Protocol. A
router's Hello Packet contains its Router Priority, which is
configurable on a per-interface basis. In general, when a
router's interface to a network first becomes functional, it
checks to see whether there is currently a Designated Router for
the network. If there is, it accepts that Designated Router,
regardless of its Router Priority. (This makes it harder to
predict the identity of the Designated Router, but ensures that
the Designated Router changes less often. See below.)
Otherwise, the router itself becomes Designated Router if it has
the highest Router Priority on the network. A more detailed
(and more accurate) description of Designated Router election is
presented in Section 9.4.
The Designated Router is the endpoint of many adjacencies. In
order to optimize the flooding procedure on broadcast networks,
the Designated Router multicasts its Link State Update Packets
to the address AllSPFRouters, rather than sending separate
packets over each adjacency.
Section 2 of this document discusses the directed graph
representation of an area. Router nodes are labelled with their
Router ID. Multi-access network nodes are actually labelled
with the IP address of their Designated Router. It follows that
when the Designated Router changes, it appears as if the network
node on the graph is replaced by an entirely new node. This
will cause the network and all its attached routers to originate
new link state advertisements. Until the topological databases
again converge, some temporary loss of connectivity may result.
This may result in ICMP unreachable messages being sent in
response to data traffic. For that reason, the Designated
Router should change only infrequently. Router Priorities
should be configured so that the most dependable router on a
network eventually becomes Designated Router.
7.4. The Backup Designated Router
In order to make the transition to a new Designated Router
smoother, there is a Backup Designated Router for each multi-
access network. The Backup Designated Router is also adjacent
to all routers on the network, and becomes Designated Router
when the previous Designated Router fails. If there were no
Backup Designated Router, when a new Designated Router became
necessary, new adjacencies would have to be formed between the
new Designated Router and all other routers attached to the
network. Part of the adjacency forming process is the
synchronizing of topological databases, which can potentially
take quite a long time. During this time, the network would not
be available for transit data traffic. The Backup Designated
obviates the need to form these adjacencies, since they already
exist. This means the period of disruption in transit traffic
lasts only as long as it takes to flood the new link state
advertisements (which announce the new Designated Router).
The Backup Designated Router does not generate a network links
advertisement for the network. (If it did, the transition to a
new Designated Router would be even faster. However, this is a
tradeoff between database size and speed of convergence when the
Designated Router disappears.)
The Backup Designated Router is also elected by the Hello
Protocol. Each Hello Packet has a field that specifies the
Backup Designated Router for the network.
In some steps of the flooding procedure, the Backup Designated
Router plays a passive role, letting the Designated Router do
more of the work. This cuts down on the amount of local routing
traffic. See Section 13.3 for more information.
7.5. The graph of adjacencies
An adjacency is bound to the network that the two routers have
in common. If two routers have multiple networks in common,
they may have multiple adjacencies between them.
One can picture the collection of adjacencies on a network as
forming an undirected graph. The vertices consist of routers,
with an edge joining two routers if they are adjacent. The
graph of adjacencies describes the flow of routing protocol
packets, and in particular Link State Update Packets, through
the Autonomous System.
Two graphs are possible, depending on whether the common network
is multi-access. On physical point-to-point networks (and
virtual links), the two routers joined by the network will be
adjacent after their databases have been synchronized. On
multi-access networks, both the Designated Router and the Backup
Designated Router are adjacent to all other routers attached to
the network, and these account for all adjacencies.
These graphs are shown in Figure 10. It is assumed that Router
RT7 has become the Designated Router, and Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated
Router performs a lesser function during the flooding procedure
than the Designated Router (see Section 13.3). This is the
reason for the dashed lines connecting the Backup Designated