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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 link-state or SPF ...
... encapsulated in
any further protocol headers as they transit the Autonomous
System. OSPF is a dynamic routing protocol. It quickly detects
...
... router maintains a
database describing the Autonomous System's topology. Each
participating router ...
... route to each destination in the Autonomous System. Externally
derived routing information appears on the tree ...
... 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 ...
... 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 a single IGP ...
... Autonomous system. Abbreviated as IGP. Each Autonomous
System has a single IGP. Separate Autonomous Systems may be
...
... IGP. Each Autonomous
System has a single IGP. Separate Autonomous Systems may be
running different IGPs.
...
... OSPF
protocol. This number uniquely identifies the router within
an Autonomous System.
Network ...
...
The Autonomous System's topological database 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 ...
... When no OSPF areas are configured, each router in the Autonomous
System has an identical topological database, leading to an
...
... N10 N7
Figure 2: A sample Autonomous System
...
... 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
...
... 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 ...
... 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
link-state domain ...
...
Routing in the Autonomous System takes place on two levels,
depending on whether the source and destination of a packet reside
...
... 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 each of the areas as spokes.
...
... routing information with routers
belonging to other Autonomous Systems. Such a router has AS
...
... AS
external routes that are advertised throughout the
Autonomous System. The path to each AS boundary router is
...
... Router RT1 can decide between
RT5 or RT7 when sending to a destination in another Autonomous
System (one of the networks N12-N15).
...
... backbone itself must not
partition. If it does, parts of the Autonomous System will
become unreachable. Backbone partitions ...
... 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 ...
... 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
...
... additional 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 ...
... 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 (such as EGP ...
... 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 ...
... Link State Update Packets, 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
...
...
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
...
... 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 by examining the LS sequence, LS checksum ...
... AS external link advertisements are
flooded throughout the entire Autonomous System, excepting
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 ...
... itself before its last restart/reload, still exists in the
Autonomous System. For more information see Section 13.4.
...
... destination described is
one that is external to the area, yet still belonging to the
Autonomous System.
Summary link ...
... link advertisements describe routes to
destinations external to the Autonomous System. Most AS
external link ...
... bits set; see Appendix F for
details). However, a default route for the Autonomous
System can be described in an AS external link advertisement
...
... 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.
...
... non-zero, it
should point to a router belonging to another Autonomous
System.
A forwarding address ...
... backbone area (Area ID = 0.0.0.0) cannot be disconnected,
or some areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links ...
... to other areas.
As an example of the calculation, consider the Autonomous System
pictured in Figure 17. There is a single non-backbone area
...
... link advertisement can also describe a default route for the
Autonomous System (Destination ID = DefaultDestination,
network ...
... address is non-zero, it should point to a router belonging to another Autonomous System. See Section 12.4.5 for more details. ...
... 32-bit number that uniquely identifies the router
in the Autonomous System. One algorithm for Router ID
...