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Routing Terms

To understand routing, become familiar with the terms defined in Table 89.

Table 89: Routing Terms




Portion of the local routing information that pertains to the reachability of a single neighbor over a single circuit or interface.


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.

area border router (ABR)

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.

AS path

In BGP, the list of autonomous system (ASs) that a packet must traverse to reach a given set of destinations within a single AS.

autonomous system (AS)

Network, collection of routers, or portion of a large internetwork under a single administrative authority.

backbone area

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

bidirectional connectivity

Ability of directly connected devices to communicate with each other over the same link.

Border Gateway Protocol (BGP)

Exterior gateway protocol used to exchange routing information among routers in different autonomous systems.


Operation of sending network traffic from one network node to all other network nodes.


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.


In BGP, a group of autonomous systems (ASs) that appears to external ASs to be a single AS.

confederation sequence

Ordered set of autonomous systems (ASs) for a confederation. The closest AS in the path is first in the sequence.


After a topology change, the time all the routers in a network take to receive the information and update their routing tables.


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.

designated router (DR)

In OSPF, a node designated to process link-state advertisements (LSAs) and distribute topology updates for an autonomous system (AS).

distance vector

Number of hops to a routing destination.

dynamic routing

Routing method that enables the route of a message through a network to change as network conditions change. Compare static routing.

end systems

Network entities that send and receive packets.

exterior gateway protocol (EGP)

Protocol that exchanges routing information between autonomous systems (ASs). BGP is an EGP. Compare interior gateway protocol (IGP).

external BGP (EBGP)

BGP configuration in which sessions are established between routers in different autonomous systems (ASs).

external peer

In BGP, a peer that resides in a different autonomous system (AS) from the Services Router.

external route

Route to an area outside the network.


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.

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.

full mesh

Network in which devices are organized in a mesh topology, with each node connected to every other network node.

gateway router

Node on a network that serves as an entrance to another network.

global AS

Global autonomous system (AS). An AS consisting of multiple subautonomous systems (sub-ASs).


Process of exchanging signaling information between two communications devices to establish the method and transmission speed of a connection.

hello packet

In OSPF, a packet sent periodically by a router to first establish and then maintain network adjacency, and to discover neighbor routers.

hold time

Maximum number of seconds allowed to elapse between the time a BGP system receives successive keepalive or update messages from a peer.


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.

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.

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.

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).

Internal BGP (IBGP)

BGP configuration in which sessions are established between routers in the same autonomous systems (ASs).

internal peer

In BGP, a peer that resides in the same autonomous system (AS) as the Services Router.

keepalive message

Periodic message sent by one BGP peer to another to verify that the session between them is still active.


Delay that occurs when a packet or signal is transmitted over a communications system.

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.

local preference

Optional BGP path attribute carried in internal BGP update packets that indicates the degree of preference for an external route.


Network topology in which devices are organized in a manageable, segmented manner with many, often redundant, interconnections between network nodes. See also full mesh.


Numerical value that determines how quickly a packet can reach its destination. See also cost.

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.


Adjacent router interface. A node can directly route packets to its neighbors only. See also peer.


Series of nodes interconnected by communication paths.

network diameter

Maximum hop count in a network.

network topology

Arrangement of nodes and connections in a network.


Connection point that operates as a redistribution point or an end point in a network, recognizing data transmissions and either forwarding or processing them.

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.

not-so-stubby area (NSSA)

In OSPF, a type of stub area in which external route advertisements can be flooded.

open message

Message sent between BGP peers to establish communication.

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).


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.

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.


Immediately adjacent router with which a protocol relationship has been established. See also neighbor.


The practice of exchanging Internet traffic with directly connected peers according to commercial and contractual agreements.

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.

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.


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.


In BGP, the feasibility of a route.


Scheduling algorithm in which items have the same priority and are handled in a fixed cyclic order.

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).

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.

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.

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.

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.

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.

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.

stub area

In OSPF, an area through which or into which autonomous system (AS) external route advertisements are not flooded.

subautonomous system (sub-AS)

Autonomous system (AS) members of a BGP confederation.


Subdivision of a network, which functions exactly like a network except that it has a more specific address and subnet mask (destination prefix).

three-way handshake

Process by which two routers synchronize protocols and establish a bidirectional connection.

topology database

Map of connections between the nodes in a network. The topology database is stored in each node.

triggered update

In a network that uses RIP, a routing update that is automatically sent whenever routing information changes.

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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