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| 1.4.- The
LS-database |
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| Type of routers |
| OSPF recognize three type of routers: |
| Internal (Core) Routers: A router with all
directly connected networks belonging to the same area. |
| Area Border Routers (ABR): A router that
attaches to multiple areas. These routers run multiple copies of the basic
algorithm, one copy for each attached area. |
| Backbone Routers: A router that has an
interface to the backbone area. |
| AS Boundary Routers (ASBR): A router that
exchanges routing information with routers belonging to other Autonomous
Systems (AS). These routers advertise AS external routing
information throughout the Autonomous System. |
| Sample AS |
| Figure 6 below shows a sample AS with
three areas A1, A2 and A3. Area 1 comprends
networks N1 to N4 and routers RT1 to RT4.
Area 2 comprends networks N6 to N8 and routers RT7,
RT8, RT10 and RT11. Area 3 comprends networks
N9 to N11, host H1 and routers RT9, RT11 and
RT12. Area 0 (backbone area) comprends routers RT3,
RT4, RT5, RT6, RT7, RT10 and RT11. |
| Routers RT1, RT2, RT5,
RT6, RT8, RT9 and RT12 are internal routers.
Routers RT3, RT4, RT7, RT10 and RT11 are
area border routers. Routers RT5 and RT7 are AS
boundary routers. |
| There is a virtual link (VL)
between routers RT10 and RT11. Area A3 is configured as
an stub area. |
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| The LS-database |
| Using this example let's see how, in each
router, one
LS-database is built for each different area the router belongs to, using flooding information
obtained by listening to other routers. This database will be exactly the
same for all core (internal) routers belonging to a same area. Each
area border router will have one of this databases for each area
to which the router belongs to. |
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For areas A1, A2, and A3,
i.e. not being a backbone area, they build two different views
of the AS. Let's take the area A1 as example. First, area
A1 build its own view (of the area) that it exports from the area to
the backbone through its area border routers; then, it also build
the entire view of the AS, from data that it imports from the
backbone to the area, also through its area border routers. Then,
ABRs act as an interchange mechanism of routing information between
the backbone and the different areas that compose the AS. |
|
| Let's see how areas build their own view part
of the LS-database; for the area A1 we have: |
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| This view is a table of costs FROM
every area border router belonging to the area TO every network belonging to the
area. |
| For example, costs of 4 from router
RT3 to network N1 is as follows: 1 from RT3 to the
network N3; +0 from network N3 to router RT1
(costs from networks to routers are zero); +3 from router RT1
to network N1. |
| The own view part of the LS-database for
area A2 is: |
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| The area A3 is an special case because
it is configured as an stub area. An stub area is considered
as having an unique cost, being this the cost of the farest element of the
area. For area A3 this is the host H1. Then, the cost from
router RT11 (unique area 3's ABR) to the whole area is 11,
i.e., the cost for going from router RT11 to host H1. The
table is as follows: |
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| The information from the 3 areas is
injected into the backbone by the ABRs of each area. With this
information the backbone's LS-database is built. |
| |
| Sorry, I'm
re-summarizing this theme because it is very important.. |
| An LS-database is a table of costs FROM
every router belonging to the area and every router belonging to
the AS that is advertising external routes, TO all these routers
and TO every network belonging to the AS. These networks
include those individual hosts that are specifically being advertised by the
OSPF protocol in the AS. |
| Next figure represents the Area 1's
LS-database: |
|
|
| This database is the complete view of Internet
for the two Area 1 core routers RT1 and RT2. |
| Area border routers RT3 and RT4
are in charge to advertise into Area 1 the distances to all
destinations external to the area. These include all routes to destinations
within the AS and all routes to destinations external to
the AS throughout the AS boundary routers RT5 and
RT7. |
| AS-external-LSAs are flooded to the
entire AS from the AS boundary routers RT5 and RT7.
These LSAs are also included in the Area 1's database and
yield routes to networks N12-N15. |
| Now, from the Area 1 to the outside,
area border routers RT3 and RT4 are in charge to summarize
Area 1 topology and flood it to the entire AS. These summaries
are shown in the following figure: |
|
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| Here, all networks belonging to the area are
advertised and the distance from these networks to the routers RT3
and RT4 respectively. |
| Now that we know how an area LS-database
is formed, it's time to see how the Area 0 - backbone's LS-database
is formed. This database is showed in this figure: |
|
|
| Routers in this database are only the
backbone's routers. Router RT11 belongs to the backbone
because a virtual link has been configured between routers RT10
and RT11. Area border routers RT3, RT4, RT7,
RT10 and RT11, condense the routing information of their attached
non-backbone areas for distribution via the backbone. |
| 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. |
| We are going to attempt an explanation using
the routers RT3 and RT4 as example. The routers first
calculate the SPF tree for the backbone. This gives the
distance to all other area border routers. This calculation is shown
in the next table: |
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| Next, by looking at the area summaries
from the routers in the first calculation, RT3 and RT4 can
determine the distances to all networks outside their area. These distances
are then advertised internally to the Area 1 by RT3 and RT4.
These advertisements are shown in the following table: |
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| The information exported to the Area 1
by routers RT3 and RT4 enables an internal Area 1
router, such as RT1, to choose an area border router intelligently.
It would use RT4 to reach N6, RT3 to reach N10
and would load half share between both (RT3 and RT4) for
traffic to N8. Also, RT1 can determine the shortest path
to the area border routers RT5 and RT7. Then, by looking at
RT5 and RT7's AS-external-LSAs, RT1 can decide between
RT5 and RT7 when trying to reach a network in another AS
(like N12 to N15). |
| Our AS has a weak that should be
subsanated. A fail in the link between RT6 and RT7 disconnect
the backbone. Configuring a virtual link through the Area 2
between RT7 and RT10 will give the backbone more
connectivity and more resistance to such failure. |
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