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.
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.
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.
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.
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.
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 in 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.
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.
With IP routing protocols, the general concept that a routing protocol update lists a subset of all known routes.
With routing protocols, the concept that the routing protocol advertises routes in a routing update on a regular periodic basis. This is typical of distance vector routing protocols.
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 poison 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.
A Layer 3 protocol that defines a packet that can be routed, such as IPv4 and IPv6
A set of messages and processes with which routers can exchange information about routes to reach subnets in a particular network. Examples include EIGRP, OSPF, and RIP
A distance vector routing technique in which information about routers is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.
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.
In OSPF, a neighbor state that implies that the router has exchanged Hellos with the neighbor, and all required parameters match.
Area Border Router (ABR)
A router using OSPF in which the router has interfaces in multiple OSPF areas.
Autonomous System Border Router (ASBR)
A router using OSPF in which the router learns routes via another source, typically another routing protocol, exchanging routes that are external to OSPF with the OSPF domain.
Backup Designated Router
An OSPF router connected to a multiaccess network that monitors the work of the designated router (DR) and takes over the work of the DR if the DR fails.
An OSPF packet type that lists brief descriptions of the LSAs in the OSPF LSFB.
In OSPF, a timer used for each neighbor. A router considers the neighbor to have failed if no Hellos are received from that neighbor in the time defined by the timer
In OSPF, on a multiaccess network, the router that wins an election and is therefore responsible for managing a streamlined process for exchanging OSPF topology information between all routers attached to that network.
In OSPF, a neighbor state that implies that the two routers have exchanged the complete (full) contents of their respective LSDBs
In OSPF, a characterization of the state of a neighbor in which the two neighbors have reached the full state
With OSPF and EIGRP, an interface timer that dictates how often the router should send Hello messages
An OSPF packet used to ask a neighboring router to send a particular LSA.
An OSPF packet used to send an LSA to a neighboring router
In routing protocols, another router with which a router decides to exchange routing information
For OSPF and EIGRP, a list of routers that have reached neighbor state
router ID (RID)
In OSPF, a 32-bit number, written in dotted decimal, that uniquely identifies each router
The structured data that describes the network topology to a routing protocol. Link-state and balanced hybrid routing protocols use topology tables, from which they build the entries in the routing table.
In EIGRP, when a router has learned of multiple routes to reach one subnet, if the best route's metric is X, the feasibility condition is another route whose reported distance is <= X
In EIGRP, the metric of the best route to reach a subnet
In EIGRP, a route that is not the best route (successor route) but that can be used immediately if the best route fails, without causing a loop. Such a route meets the feasibility condition
From one EIGRP router's perspective, the metric for a subnet as calculated on a neighboring router and reported in a routing update to the first router.
In EIGRP, the route to reach a subnet that has the best metric and should be placed in the IP routing table.