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autonomous system
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... routing information
between routers belonging to a single Autonomous System. The OSPF
protocol is based on SPF or link-state ...
... encapsulated in any further protocol headers as
they transit the Autonomous System. OSPF is a dynamic routing protocol.
...
... router maintains a database
describing the Autonomous System's topology. Each participating router
...
... gives the route to each
destination in the Autonomous System. Externally derived routing
information appears on the tree as leaves.
...
... to be grouped together. Such a grouping is
called an area. The topology of an area is hidden from the rest of the
Autonomous System. This information hiding enables a significant
reduction in routing traffic ...
... authenticated. This means that only
trusted routers can participate in the Autonomous System's routing. A
variety of authentication ...
... Gateway Protocol (EGP)) is passed transparently throughout the
Autonomous System. This externally derived data is kept separate from
the OSPF protocol's link state ...
... router, enabling the passing of additional
information between routers on the boundaries of the Autonomous System.
...
... routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous System has
...
... Autonomous system. Abbreviated as IGP. Each Autonomous System has
a single IGP. Different Autonomous Systems ...
... Autonomous System has
a single IGP. Different Autonomous Systems may be running different
IGPs.
...
... OSPF protocol.
This number uniquely identifies the router within an Autonomous
System.
Network
...
...
The database of the Autonomous System's topology describes a directed
graph. The vertices of the graph consist of routers ...
... node.
Figure 2 shows a sample map of an Autonomous System. The rectangle
labelled H1 indicates a host, which has a SLIP ...
... EGP connections
to other Autonomous Systems. A set of EGP-learned routes have been
displayed for both of these routers ...
... version.)
Figure 2: A sample Autonomous System
______________________________________
...
... When no OSPF areas are configured, each router in the Autonomous System
has an identical topological database, leading to an identical graphical
...
... such as EGP, or be statically configured (static routes).
Default routes can also be included as part of the Autonomous System's
external routing information.
...
... AS. In
our example, all the routers in the Autonomous System know that router
RT7 has two external routes, with metrics 2 and 9.
...
... address" has one other application. It enables routers
in the Autonomous System's interior to function as "route servers". For
example, in Figure 2 the router ...
... OSPF external advertisements. In each external
advertisement, router RT6 would specify the correct Autonomous System
exit point to use for the destination through appropriate setting of the
...
... routing traffic as
compared to treating the entire Autonomous System as a single SPF
domain.
...
... .
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
...
... The backbone of the Autonomous System ...
... inter-area routing can be pictured as
forcing a star configuration on the Autonomous System, with the backbone
as hub and and each of the areas as spokes.
...
... routing information with routers belonging
to other Autonomous Systems. Such a router has AS external routes
...
... router has AS external routes
that are advertised throughout the Autonomous System. The path to
each AS boundary router ...
... router RT1 can decide between RT5 or RT7 when
sending to a destination in another Autonomous System (one of the
networks N12-N15).
...
...
In some Autonomous Systems, the majority of the topological database may
consist of external advertisements. An OSPF ...
... . Also, the
backbone itself must not partition. If it does, parts of the Autonomous
System will become unreachable. Backbone partitions can be repaired by
...
...
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 ...
... 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 ...
... (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 ...
... routing information into
the area. This additional information is a distillation of the rest of
the Autonomous System's topology.
...
...
Routers that have information regarding other Autonomous Systems can
flood this information throughout the AS. This external ...
... link state
advertisements. This enables routers supporting a mix of optional
capabilities to coexist in a single Autonomous System.
Some capabilities must be supported by all routers ...
... 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 ...
... routers. They comprise routes to destinations
external to the Autonomous System. Note that, if the router is
itself an AS boundary ...
... area border
routers. They describe routes to destinations internal to the
Autonomous System, yet external to the area.
Shortest-path tree ...
... routing protocol packets, and in particular Link State
Updates, through the Autonomous System.
Two graphs are possible, depending on whether the common network ...
...
Consider the Autonomous System pictured in Figure 2. No OSPF areas have
been configured. A single metric is shown per outbound interface ...
...
Each router in the Autonomous System originates one or more link state
advertisements. There are five distinct types of link state
advertisements, which are described in Section 4.3. The collection of
...
... advertisements provide a way of transparently advertising externally-
derived routing information throughout the Autonomous System.
Each link state advertisement ...
... .
There may be several instances of an advertisement present in the
Autonomous System, all at the same time. It must then be determined
which instance is more recent. This determination is made be examining
the LS sequence, LS checksum ...
... AS external link advertisements are
flooded throughout the entire Autonomous System, excluding stub areas
(see Section 3.6). Each separate advertisement type is briefly
described below in Table 15.
...
... to destinations external to the
Autonomous System. A default route for
the Autonomous System ...
... Autonomous System. A default route for
the Autonomous System can also be
described by an AS external link ...
... router itself before its last restart/reload, still
exists in the Autonomous System. For more information see Section 13.4.
,uh "12.1.7 LS checksum ...
... single area only. The destination described is one that is external to
the area, yet still belonging to the Autonomous System.
The DefaultDestination can also be specified in summary link ...
... link advertisements describe routes to destinations external
to the Autonomous System. Most AS external link advertisements describe
...
... destinations. However, a default route for
the Autonomous System can be described in an AS external advertisement
by setting the advertisement's Link State ...
... AS external link advertisements are the only type of link
state advertisements that are flooded throughout the entire Autonomous
System; all other types of link state advertisements are specific to a
single area. However, AS ...
... non-zero, it should point
to a router belonging to another Autonomous System.
A forwarding address ...
... backbone area (Area ID = 0) cannot be disconnected, or some
areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links ...
... traffic will take.
As an example, consider the Autonomous System pictured in Figure 17.
There is a single non-backbone area (Area 1) that physically divides the
...
... AS external advertisement can also
describe a default route for the Autonomous System (destination =
DefaultDestination). For each AS ...
... address is non-zero, it should point to a router belonging to another Autonomous System. See Section 12.4.4 for more details. ...
... 32-bit number that uniquely identifies the router in the
Autonomous System. One algorithm for Router ID assignment is to
...
... database
consists of AS external advertisements. In these Autonomous Systems,
some OSPF areas may be organized in such a way that external
...
... servers". Using the forwarding
address, a router in the middle of the Autonomous System can gather
external routing information and originate AS ...