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adjacency
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Portion of the local routing information that pertains to the reachability
of a single neighbor over a single circuit or interface.
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area
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Administrative group of OSPF networks within an autonomous system (AS)
that operates independently from other areas in the AS. Multiple areas within
an AS reduce the amount of link-state advertisement (LSA) traffic on the network
and the size of topology databases.
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area border router (ABR)
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In OSPF, a router having interfaces in multiple areas of an autonomous
system (AS) so that it can link the areas to each other. An area border router
maintains a separate topological database for each area it is connected to
and shares topology information between areas.
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AS path
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In BGP, the list of autonomous system (ASs) that a packet must traverse
to reach a given set of destinations within a single AS.
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autonomous system (AS)
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Network, collection of routers, or portion of a large internetwork under
a single administrative authority.
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backbone area
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In OSPF, the central area in an autonomous system (AS) to which all
other areas are connected by area border routers (ABRs). The backbone area
always has the area ID 0.0.0.0.
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bidirectional connectivity
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Ability of directly connected devices to communicate with each other
over the same link.
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Border Gateway Protocol (BGP)
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Exterior gateway protocol used to exchange routing information among
routers in different autonomous systems.
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broadcast
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Operation of sending network traffic from one network node to all other
network nodes.
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cluster
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In BGP, a set of routers that have been grouped together. A cluster
consists of one system that acts as a route reflector, along with any number
of client peers. The client peers receive their route information only from
the route reflector system. Routers in a cluster do not need to be fully meshed.
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confederation
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In BGP, a group of autonomous systems (ASs) that appears to external
ASs to be a single AS.
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confederation sequence
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Ordered set of autonomous systems (ASs) for a confederation. The closest
AS in the path is first in the sequence.
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convergence
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After a topology change, the time all the routers in a network take
to receive the information and update their routing tables.
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cost
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Unitless number assigned to a path between neighbors, based on throughput,
round-trip time, and reliability. The sum of path costs between source and
destination hosts determines the overall path cost. OSPF uses the lowest cost
to determine the best path.
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designated router (DR)
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In OSPF, a node designated to process link-state advertisements (LSAs)
and distribute topology updates for an autonomous system (AS).
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distance vector
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Number of hops to a routing destination.
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dynamic routing
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Routing method that enables the route of a message through a network
to change as network conditions change. Compare static routing.
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end systems
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Network entities that send and receive packets.
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exterior gateway protocol (EGP)
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Protocol that exchanges routing information between autonomous systems
(ASs). BGP is an EGP. Compare interior gateway protocol (IGP).
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external BGP (EBGP)
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BGP configuration in which sessions are established between routers
in different autonomous systems (ASs).
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external peer
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In BGP, a peer that resides in a different autonomous system (AS) from
the Services Router.
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external route
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Route to an area outside the network.
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flooding
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Technique by which a router forwards traffic to every node attached
to the router, except the node from which the traffic arrived. Flooding is
a simple but sometimes inefficient way to distribute routing information quickly
to every node in a network. RIP and OSPF are flooding protocols, but BGP is
not.
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forwarding table
|
JUNOS software forwarding information base (FIB). The JUNOS routing
protocol process installs active routes from its routing tables into the Routing
Engine forwarding table. The kernel copies this forwarding table into the
Packet Forwarding Engine, which is responsible for determining which interface
transmits the packets.
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full mesh
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Network in which devices are organized in a mesh topology, with each
node connected to every other network node.
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gateway router
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Node on a network that serves as an entrance to another network.
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global AS
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Global autonomous system (AS). An AS consisting of multiple subautonomous
systems (sub-ASs).
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handshake
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Process of exchanging signaling information between two communications
devices to establish the method and transmission speed of a connection.
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hello packet
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In OSPF, a packet sent periodically by a router to first establish and
then maintain network adjacency, and to discover neighbor routers.
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hold time
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Maximum number of seconds allowed to elapse between the time a BGP system
receives successive keepalive or update messages from a peer.
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hop
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Trip a data packet takes from one router to another in the network.
The number of routers through which a packet passes to get from its source
to its destination is known as the hop count. In general, the best route is
the one with the shortest hop count.
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intermediate systems
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Network entities that relay (forward) packets as well as send and receive
them on the network. Intermediate systems are also known as routers.
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Intermediate System-to-Intermediate System (IS-IS)
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Link-state, interior gateway routing protocol for IP networks that also
uses the shortest-path-first (SPF) algorithm to determine routes.
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interior gateway protocol (IGP)
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Protocol that exchanges routing information within autonomous systems
(ASs). IS-IS, OSPF, and RIP are IGPs. Compare exterior gateway protocol
(EGP).
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Internal BGP (IBGP)
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BGP configuration in which sessions are established between routers
in the same autonomous systems (ASs).
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internal peer
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In BGP, a peer that resides in the same autonomous system (AS) as the
Services Router.
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keepalive message
|
Periodic message sent by one BGP peer to another to verify that the
session between them is still active.
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latency
|
Delay that occurs when a packet or signal is transmitted over a communications
system.
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link-state advertisement (LSA)
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Messages that announce the presence of OSPF-enabled interfaces to adjacent
OSPF interfaces (neighbors). The exchange of LSAs establishes bidirectional
connectivity between neighbors.
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local preference
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Optional BGP path attribute carried in internal BGP update packets that
indicates the degree of preference for an external route.
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mesh
|
Network topology in which devices are organized in a manageable, segmented
manner with many, often redundant, interconnections between network nodes.
See also full mesh.
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metric
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Numerical value that determines how quickly a packet can reach its destination.
See also cost.
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multiple exit discriminator (MED)
|
Optional BGP path attribute consisting of a metric value that is used
to determine the exit point to a destination when all other factors in determining
the exit point are equal.
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neighbor
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Adjacent router interface. A node can directly route packets to its
neighbors only. See also peer.
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network
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Series of nodes interconnected by communication paths.
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network diameter
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Maximum hop count in a network.
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network topology
|
Arrangement of nodes and connections in a network.
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node
|
Connection point that operates as a redistribution point or an end point
in a network, recognizing data transmissions and either forwarding or processing
them.
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notification message
|
Message sent between BGP peers to inform the receiving peer that the
sending peer is terminating the session because an error occurred, and explaining
the error.
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not-so-stubby area (NSSA)
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In OSPF, a type of stub area in which external route advertisements
can be flooded.
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open message
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Message sent between BGP peers to establish communication.
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Open Shortest Path First protocol (OSPF)
|
A link-state interior gateway protocol (IGP) that makes routing decisions
based on the shortest-path-first (SPF) algorithm (also referred to as the
Dijkstra algorithm).
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origin
|
Value assigned to a BGP route to indicate whether the first router to
advertise the route learned it from an external, internal, or unknown source.
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path-vector protocol
|
Protocol that uses the path between autonomous systems (ASs) to select
the best route, rather than the shortest distance or the characteristics of
the route (link state). BGP is a path-vector protocol. In contrast, RIP is
a distance-vector protocol, and OSPF and IS-IS are link-state protocols.
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peer
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Immediately adjacent router with which a protocol relationship has been
established. See also neighbor.
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peering
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The practice of exchanging Internet traffic with directly connected
peers according to commercial and contractual agreements.
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point of presence (POP)
|
Access point to the Internet, having a unique IP address, where telecommunications
equipment is located. POPs usually belong to Internet service providers (ISPs)
or telephone companies.
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poison reverse
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An efficiency technique in a RIP network. By setting the number of hops
to an unavailable router to 16 hops or more, a router informs all the other
routers in the network. Because RIP allows only up to 15 hops to another router,
this technique reduces RIP updates and helps defeat large routing loops. See
also split horizon.
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propagation
|
Process of translating and forwarding route information discovered by
one routing protocol in the update messages of another routing protocol. Route
propagation is also called route redistribution.
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reachability
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In BGP, the feasibility of a route.
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round-robin
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Scheduling algorithm in which items have the same priority and are handled
in a fixed cyclic order.
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route advertisement
|
Distribution of routing information at specified intervals throughout
a network, to establish adjacencies with neighbors and communicate usable
routes to active destinations. See also link-state advertisement
(LSA).
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route aggregation
|
Combining groups of routes with common addresses into a single entry
in the routing table, to decrease routing table size and the number of route
advertisements sent by a router.
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route reflection
|
In BGP, configuring a group of routers into a cluster and having one
system act as a route reflector, redistributing routes from outside the cluster
to all routers in the cluster. Routers in a cluster do not need to be fully
meshed.
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Routing Information Protocol (RIP)
|
Distance-vector routing protocol that keeps a database of routing information
gathered from periodic broadcasts by each router in a network.
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routing table
|
Table stored on a router that keeps track of all possible paths (routes)
between sources and destinations in a network and, in some cases, metrics
associated with the routes.
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split horizon
|
An efficiency technique in a RIP network. A router reduces the number
of RIP updates in the network by not retransmitting a route advertisement
out the interface through which it was received. Split-horizon updates also
help prevent routing loops. See also poison reverse.
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static routing
|
Routing method in which routes are manually entered in the routing table
and do not change unless you explicitly update them. Unlike dynamic routes,
which must be imported into the routing table each time a host comes online,
static routes are available immediately. Static routes are generally preferred
over other types of routes. Compare dynamic routing.
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stub area
|
In OSPF, an area through which or into which autonomous system (AS)
external route advertisements are not flooded.
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subautonomous system (sub-AS)
|
Autonomous system (AS) members of a BGP confederation.
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subnetwork
|
Subdivision of a network, which functions exactly like a network except
that it has a more specific address and subnet mask (destination prefix).
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three-way handshake
|
Process by which two routers synchronize protocols and establish a bidirectional
connection.
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topology database
|
Map of connections between the nodes in a network. The topology database
is stored in each node.
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triggered update
|
In a network that uses RIP, a routing update that is automatically sent
whenever routing information changes.
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virtual link
|
In OSPF, a link you create between two area border routers (ABRs) that
have an interface to a common nonbackbone area, to connect a third area to
the backbone area. One of the area border routers must be directly connected
to the backbone area.
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