Cisco - Basic Terminology for EIGRP

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Cisco - Basic Terminology for EIGRP
Table of Contents
Basic Terminology for EIGRP...........................................................................................................................1
Introduction..............................................................................................................................................1
Prerequisites................................................................................................................................1
Frequently Used EIGRP Terms...............................................................................................................1
DUAL.........................................................................................................................................1
Feasible Distance........................................................................................................................1
Feasibility Condition...................................................................................................................2
Successor....................................................................................................................................2
Feasible Successor......................................................................................................................2
Active State.................................................................................................................................2
Passive State...............................................................................................................................2
Stuck-in-Active.........................................................................................................................2
Hello/ACKs................................................................................................................................2
Update.........................................................................................................................................2
Query..........................................................................................................................................3
Reply...........................................................................................................................................3
Neighbor Table...........................................................................................................................3
Topology Table...........................................................................................................................3
Related Information.................................................................................................................................3
Cisco - Basic Terminology for EIGRP
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Basic Terminology for EIGRP
Introduction
Prerequisites
Frequently Used EIGRP Terms
DUAL
Feasible Distance
Feasible Condition
Successor
Feasible Successor
Active State
Passive State
Stuck-in-Active
Hello/ACKs
Update
Query
Reply
Neighbor Table
Topology Table
Related Information
Introduction
This document explains the basic terminologies specific to Enhanced Interior Gateway Routing Protocol
(EIGRP). EIGRP is an enhancement of Interior Gateway Routing Protocol (IGRP). Both EIGRP and IGRP
are Cisco-specific protocols.
Prerequisites
This document requires an understanding of routing protocols. To learn more about routing protocols, please
refer to Routing Basics. After reading Routing Basics, you will have a better knowledge of the common
routing terms mentioned in this document.
Frequently Used EIGRP Terms
DUAL
Diffusing Update Algorithm (DUAL) is the algorithm used by EIGRP for optimum path determination.
DUAL is designed to take into consideration that even temporary routing loops cause great losses in
performance levels in an internetwork. DUAL uses diffusing computations to determine the shortest path to a
destination while remaining loop-free at every instance.
Feasible Distance
Feasible distance (FD) is the lowest calculated metric for a destination. For example, if there are two paths
with metrics of 12466 and 11200, then the FD is 11200 to reach a destination
Cisco - Basic Terminology for EIGRP
Feasibility Condition
Feasibility condition (FC) is met when the neighbor's FD, also known as reported distance, to a destination is
lower than the router's current FD to the same destination.
Successor
A successor is a neighbor that has been selected as the next hop for a destination based on the FC. Only the
successors are installed in the routing table.
Feasible Successor
A feasible successor (FS) is a neighbor that has satisfied the FC and has a path to the destination. Feasible
successors will become successors if the router looses all its successors to a destination.
Active State
A route is said to be in active state when it has lost its only successor to a destination and has no other FS
available. The router is forced to compute a new route to the destination. The route will remain in active state
until a new successor is found, or until it is determined that a route to the destination no longer exists.
Passive State
Passive state is the normal state of an EIGRP route. All EIGRP routes will be in passive state if the router is
not performing diffusion computations. The route will remain in passive state if the only successor is lost, but
at least one feasible successor to that destination is available.
Stuck-in-Active
An EIGRP route is said to be stuck-in-active (SIA) if the router does not receive a reply to every query it sent
before a predetermined time interval. This interval is called the active timer and is set for three minutes by
default. If the router does not receive all of the replies for its queries before the active timer expires, the route
is declared SIA.
Hello/ACKs
Periodic hello packets are exchanged between EIGRP neighbors for neighbor discovery and recovery process.
Hello packets are multicast every five seconds on high speed links, such as Ethernet, and are unicast every 60
seconds on slow speed (T1 or slower) links. Hello packets are not acknowledged. Hello packets without data
are called acknowledgments (ACKs). ACKs are sent to acknowledge that packets were reliably received and
are unicast in nature.
Update
Update packets are used to exchange routing information between EIGRP neighbors. Based upon need, update
packets can be unicast or multicast in nature. All update packets are sent reliably and therefore must be
acknowledged.
Update packets are sent in the following instances:
Cisco - Basic Terminology for EIGRP
When a neighbor first comes up.·
When a router transitions from active to passive for a destination.·
When there is a metric change for a certain destination.·
Query
A query is sent to all neighbors when a router goes into active state for a destination and is asking for
information on that destination. Unless it receives replies back from all its neighbors, the router will remain in
active state and not start the computation for a new successor. Queries can be multicast or unicast in nature.
Reply
A reply is sent by every EIGRP neighbor which receives a query. If the neighbor does not have the
information about the destination that was queried, it queries all of its neighbors. Replies are always unicast in
nature.
Neighbor Table
Information on each neighbor is maintained in the Neighbor Table. The neighbor address, the interface on
which the neighbor was discovered, hold time for that neighbor, neighbor up-time, Smooth Round Trip Time
(SRTT), retransmit interval, and so on are stored for each neighbor.
Topology Table
The topology table stores routing information that neighbors exchange after the first hello exchange. DUAL
acts on the topology table to determine successors and FSs.
Related Information
IP Routing Top Issues·
EIGRP Support Page·
More EIGRP Technical Tips·
All contents are Copyright © 1992--2002 Cisco Systems Inc. All rights reserved. Important Notices and Privacy Statement.
Updated: Sep 20, 2002 Document ID: 13671
Cisco - Basic Terminology for EIGRP