3. Splitting the AS into Areas
-
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 section.
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 link-state domain.
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
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 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:
Internal routers
A router with all directly connected networks belonging to
the same area. 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. 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.
Backbone routers
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
boundary routers.
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
...........................
. + .
. | 3+---+ . N12 N14
. N1|--|RT1|\ 1 . \ N13 /
. | +---+ \ . 8\ |8/8
. + \ ____ . \|/
. / \ 1+---+8 8+---+6
. * N3 *---|RT4|------|RT5|--------+
. \____/ +---+ +---+ |
. + / \ . |7 |
. | 3+---+ / \ . | |
. N2|--|RT2|/1 1\ . |6 |
. | +---+ +---+8 6+---+ |
. + |RT3|------|RT6| |
. +---+ +---+ |
. 2/ . Ia|7 |
. / . | |
. +---------+ . | |
.Area 1 N4 . | |
........................... | |
.......................... | |
. N11 . | |
. +---------+ . | |
. | . | | N12
. |3 . Ib|5 |6 2/
. +---+ . +----+ +---+/
. |RT9| . .........|RT10|.....|RT7|---N15.
. +---+ . . +----+ +---+ 9 .
. |1 . . + /3 1\ |1 .
. _|__ . . | / \ __|_ .
. / \ 1+----+2 |/ \ / \ .
. * N9 *------|RT11|----| * N6 * .
. \____/ +----+ | \____/ .
. | . . | | .
. |1 . . + |1 .
. +--+ 10+----+ . . N8 +---+ .
. |H1|-----|RT12| . . |RT8| .
. +--+SLIP +----+ . . +---+ .
. |2 . . |4 .
. | . . | .
. +---------+ . . +--------+ .
. 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.
**FROM**
|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |7 |N3|
----- -------------------
RT1| | | | | | |0 |
RT2| | | | | | |0 |
RT3| | | | | | |0 |
* RT4| | | | | | |0 |
* RT5| | |14|8 | | | |
T RT7| | |20|14| | | |
O N1|3 | | | | | | |
* N2| |3 | | | | | |
* N3|1 |1 |1 |1 | | | |
N4| | |2 | | | | |
Ia,Ib| | |20|27| | | |
N6| | |16|15| | | |
N7| | |20|19| | | |
N8| | |18|18| | | |
N9-N11,H1| | |29|36| | | |
N12| | | | |8 |2 | |
N13| | | | |8 | | |
N14| | | | |8 | | |
N15| | | | | |9 | |
Figure 7: Area 1's Database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
**FROM**
|RT|RT|RT|RT|RT|RT|RT
|3 |4 |5 |6 |7 |10|11|
------------------------
RT3| | | |6 | | | |
RT4| | |8 | | | | |
RT5| |8 | |6 |6 | | |
RT6|8 | |7 | | |5 | |
RT7| | |6 | | | | |
* RT10| | | |7 | | |2 |
* RT11| | | | | |3 | |
T N1|4 |4 | | | | | |
O N2|4 |4 | | | | | |
* N3|1 |1 | | | | | |
* N4|2 |3 | | | | | |
Ia| | | | | |5 | |
Ib| | | |7 | | | |
N6| | | | |1 |1 |3 |
N7| | | | |5 |5 |7 |
N8| | | | |4 |3 |2 |
N9-N11,H1| | | | | | |11|
N12| | |8 | |2 | | |
N13| | |8 | | | | |
N14| | |8 | | | | |
N15| | | | |9 | | |
Figure 8: The backbone's database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
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 LSAs, 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.
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
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.
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 128.185.0.0 with a mask of 0xffff0000 actually is
describing a single route to the collection of destinations
128.185.0.0 - 128.185.255.255. 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 128.185.0.0 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
masks.
Network address IP address mask Subnet size
_______________________________________________
128.185.16.0 0xfffff000 4K
128.185.1.0 0xffffff00 256
128.185.0.8 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
is unambiguous.
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
[192.9.4.0,0xfffffc00]. The segment would then be single IP
network, containing addresses from the four consecutive class C
network numbers 192.9.4.0 through 192.9.7.0. 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
range.
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
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-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-
external-LSAs.
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
areas.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.[1] 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
region.