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adjacency
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The relationship between selected neighboring routers for exchanging
routing information. Not every pair of neighboring routers is adjacent.
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area
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A collection of network segments interconnected by routers.
It is a region in an OSPF routing domain.
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area border router (ABR)
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A router that sits on the edge of an OSPF area and routes link-state
advertisements (LSAs) between areas.
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area ID
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A unique number that identifies an area. Typically, formatted
as an IP address.
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authentication
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A process whereby a user or data source proves that it is what
it claims to be.
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authentication type
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The method by which authentication is achieved—null (or
none), simple, or MD5. For example, simple authentication requires
a 64-bit password in each OSPF packet.
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autonomous system (AS)
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A set of networks or IP prefixes within a single routing policy
domain.
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autonomous system boundary router
(AS boundary router)
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An OSPF router that redistributes routing information from other
routing protocol sources.
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classless interdomain routing (CIDR)
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An addressing method that replaces the traditional class structure
of IP addresses. In CIDR, the boundary between the network and host
portions of an IP address can be on any bit boundary. CIDR addresses
have no class restrictions, enabling more efficient used of the IP
address space. CIDR addresses are represented by a prefix and a notation
that indicates the IP address and mask; for example, 10.12.8.3/16.
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designated
router
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A designated device (OSPF router) with which other routers form
adjacencies, reducing the number of adjacencies required on a broadcast
or NBMA network.
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domain
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A collection of routers that use a common interior gateway protocol.
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flooding
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The distribution and synchronization of the link-state database
between OSPF routers.
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hello protocol
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A protocol that establishes and maintains neighbor relationships
and that communication between neighbors is bidirectional. The hello
protocol also dynamically discovers neighboring routers on broadcast
or point-to-point networks.
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interior gateway protocol (IGP)
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A routing protocol that routers within an AS use to exchange
information.
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link-state advertisement (LSA)
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A unit of data that describes the local state of a router or
network. LSAs are flooded throughout their respective flooding domains.
For example, router LSAs are flooded within the area to which the
router belongs, summary LSAs are flooded to other areas through the
backbone, and external LSAs are flooded throughout the OSPF domain.
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LSA types
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OSPF LSAs are categorized into the following types:
- Type 1—LSAs generated by an OSPF router for each
area that it belongs to. Type 1 LSAs are flooded to only a single
area. These LSAs carry information about directly connected links.
Also known as router LSA.
- Type 2—LSAs generated by an OSPF designated router
to describe the set of routers in a network. Type 2 LSAs are flooded
to the area that contains that network. Also known as network LSA.
- Type 3—LSAs generated by an ABR to describe inter-area
routes to networks outside of that area and internal to the AS; used
for route summarization. Also known as inter-area prefix LSA.
- Type 4—LSAs generated by an ABR to describe inter-area
routes to ASBRs outside of that area and internal to the AS; used
for route summarization. Also known as inter-area router LSA.
- Type 5—LSAs generated by an ASBR to describe links
that are external to the AS. Type 5 LSAs are reflooded from other
protocols into OSPF, and are flooded by OSPF throughout the routing
domain to all area types other than stub areas. OSPF sets the forwarding
address for a type 5 LSA when the next hop is directly connected to
the OSPF interface. Also known as AS-external LSA.
- Type 6—Not supported.
- Type 7—LSAs generated by an ASBR to describe routes
that are external to an NSSA. Type 7 LSAs are flooded only to NSSAs.
- Type 8—Not supported.
- Type 9—Opaque LSA with a link-local scope. Type
9 LSAs are not flooded beyond the local network (local link).
- Type 10—Opaque LSA with an area-local scope. Type-10
LSAs are not flooded beyond the borders of their associated area.
- Type 11—Opaque LSA flooded throughout the AS. Type
11 LSAs are flooded throughout all transit areas, are not flooded
into stub areas from the backbone, and are not originated by routers
into their connected stub areas. Any type 11 LSA received in a stub
area from a neighboring router within the stub area is rejected.
- Link LSA—OSPFv3 LSA that Provides the router’s
link-local address to all other routers attached to the link; informs
other routers attached to the link of a list of IPv6 prefixes to associate
with the link; enables the router to assert a collection of options
bits in the network LSA to be originated for the link
- Intra-area prefix LSA—OSPFv3 LSA that associates
a list of IPv6 address prefixes with a transit network link by referencing
a network LSA, or associates a list of IPv6 address prefixes with
a router by referencing a router LSA. The intra-area prefix LSA includes
the IPv6 prefix information that OSPFv2 includes in type 1 and type
2 LSAs.
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neighboring routers
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Routers that have interfaces to a common network.
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nonbroadcast network
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A network that has no broadcast capability but supports more
than two routers.
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Not-so-stubby area (NSSA)
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Similar to a stub area, but can also import selected external
LSAs.
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router ID
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A 32-bit number that uniquely identifies a router within an
AS; for example, 10.10.1.5.
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stub area
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An area that does not get flooded with external LSAs but does
carry intra-area and interarea routes and a default route.
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Totally stubby area
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A stub area that also blocks type 3 summary LSAs from flowing
into the area; however, type 3 LSAs carrying default route information
alone are injected into the area.
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virtual link
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A logical link between two backbone routers for which the link
tunnels through a nonbackbone area.
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