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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.
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.
__________________________________________
(Figure not included in text version.)
Figure 6: A sample OSPF area configuration
__________________________________________
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RFC 1247 OSPF Version 2 July 1991
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.
______________________________________
(Figure not included in text version.)
Figure 7: Area 1's Database
Figure 8: The backbone database
______________________________________
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
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RFC 1247 OSPF Version 2 July 1991
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.
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 I5 and I6 into a
single advertisement.
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
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.
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RFC 1247 OSPF Version 2 July 1991
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
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 another 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
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 subnet
masks. 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 64 variable-sized subnets:
16 subnets of size 4K, 16 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.
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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. The 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.
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 external advertisements. An OSPF external advertisement is
usually flooded throughout the entire AS. However, OSPF allows certain
areas to be configured as "stub areas". 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 advertisements. These
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RFC 1247 OSPF Version 2 July 1991
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 advertisement), instead of flooding the
external advertisements 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 external advertisements.
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.
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.[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
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