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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
|
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)
|
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
|
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
|
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
|
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
|
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
|
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
|
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)
|
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)
|
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)
|
BGP configuration in which sessions are established between
routers in the same autonomous systems (ASs).
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internal peer
|
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)
|
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
|
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
|
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
|
Adjacent router interface. A node can directly route packets
to its neighbors only. See also peer.
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network
|
Series of nodes interconnected by communication paths.
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network diameter
|
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)
|
In OSPF, a type of stub area in which external route advertisements
can be flooded.
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open message
|
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
|
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
|
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
|
In BGP, the feasibility of a route.
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round-robin
|
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 Information Protocol next generation (RIPng)
|
Distance-vector routing protocol that exchanges routing information
used to compute routes and is intended for Internet Protocol version
6 (IPv6)-based networks.
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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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