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balanced hybrid
Refers to one of three general types of routing protocol algorithms. The other two are distance vector and link-state. EIGRP is the only routing protocol that Cisco classifies as using a balanced hybrid algorithm.
convergence
The time required for routing protocols to react to changes in the network, removing bad routes and adding new, better routes so that the current best routes are in all the routers' routing tables.
counting to infinity
An unfortunate side effect of distance vector routing protocols in which the routers slowly increase the metric for a failed route until the metric reaches that routing protocol's finite definition of a maximum metric (called infinity).
Dijkstra Shortest Path First (SPF) algorithm
The name of the algorithm used by link-state routing protocols to analyze the LSDB and find the least-cost routes from that router to each subnet.
distance vector
The logic behind the behavior of some interior routing protocols, such as RIP and IGRP. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbors. Distance vector routing algorithms can be prone to routing loops but are computationally simpler than link-state routing algorithms. Also called Bellman-Ford routing algorithm.
exterior gateway protocol (EGP)
A routing protocol that was designed to exchange routing information between different autonomous systems.
full update
With IP routing protocols, the general concept that a routing protocol update lists all known routes. See also partial update.
holddown (holddown timer)
A Distance Vector protocol state assigned to a route placed so that routers neither advertise the route nor accept advertisements about it for a specific length of time (the holddown timer). Holddown is used to flush bad information about a route from all routers in the network. A route typically is placed in holddown when a link in that route fails.
infinity
In the context of IP routing protocols, a finite metric value defined by the routing protocol that is used to represent an unusable route in a routing protocol update.
interior gateway protocol (IGP)
A routing protocol designed to be used to exchange routing information inside a single autonomous system.
link-state
A classification of the underlying algorithm used in some routing protocols. Link-state protocols build a detailed database that lists links (subnets) and their state (up, down), from which the best routes can then be calculated.
link-state advertisement (LSA)
In OSPF, the name of the data structure that resides inside the LSDB and describes in detail the various components of a network, including routers and links (subnets).
link-state database (LSDB)
In OSPF, the data structure in RAM of a router that holds the various LSAs, with the collective LSAs representing the entire topology of the network.
metric
A numeric measurement used by a routing protocol to determine how good a route is as compared to other alternate routes to reach the same subnet.
partial update
With IP routing protocols, the general concept that a routing protocol update lists a subset of all known routes. See also full update.
periodic update
With routing protocols, the concept that the routing protocol advertises routes in a routing update sent on a regular periodic basis. This is typical of distance vector routing protocols.
poison reverse
A distance vector poisoned route advertisement for a subnet that would not have been advertised because of split-horizon rules but is now advertised as a poisoned route.
poisoned route
A route in a routing protocol's advertisement that lists a subnet with a special metric value, called an infinite metric, that designates the route as a failed route.
routable protocol
Same as a routed protocol.
routed protocol
A Layer 3 protocol that defines a packet that can be routed, such as IPv4 and IPv6.
routing protocol
A set of messages and processes with which routers can exchange information about routes to reach subnets in a particular network. Examples of routing protocols include Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF), and Routing Information Protocol (RIP).
split horizon
A distance vector routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.
triggered update
A routing protocol feature in which the routing protocol does not wait for the next periodic update when something changes in the network, instead immediately sending a routing update.
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