The terms and concepts presented in this appendix should prove helpful in understanding the EIGRP domain discovered and monitored by Network Protocol Manager for EIGRP.

Begin by examining Interior gateway protocol and exterior gateway protocol links.

Figure 1. Interior gateway protocol and exterior gateway protocol links

The links between the routers within an independent network, or autonomous system (AS), are referred to as Interior Gateway Protocol (IGP) links. The links between routers in different autonomous systems are referred to as Exterior Gateway Protocol (EGP) links. EIGRP is an IGP.

  • AS

    Autonomous system. A collection of networks, or more precisely, the routers joining those networks, that are under the same administrative authority and that share a common routing strategy. (Also, see EIGRP domain.)

  • ASN

    Autonomous system number. A number that uniquely identifies a particular autonomous system. An ASN is used in the exchange of exterior routing information between neighboring autonomous systems. (Also, see EIGRP process number.)

  • CIDR

    Classless Inter-Domain Routing. The standard format for designating an IP subnet. CIDR replaces the original class A, B, and C Internet-address routing scheme with an address scheme that allows a single IPv4 address to designate many unique addresses. A CIDR address is identified by an IP prefix and subnet mask; for example, CIDR is described in RFC 1519.

  • CLI

    Command line interface. A user interface, available on most networking devices, that permits a user or a software program to view statistics, make configuration changes, and perform other administrative functions. Commands are entered as text on a single line. Network Protocol Manager for EIGRP uses Telnet, SSH1, or SSH2 to log in to a Cisco EIGRP router and then enters CLI commands at the command line to query the device for EIGRP topology information and status.

  • DUAL

    Diffusing Update Algorithm. The algorithm used by EIGRP to select routing paths and to guarantee freedom from routing loops. DUAL selects a best path and a second best path to reach a destination. The best path selected by DUAL is the successor, and the second best path (if available) is the feasible successor. The feasible distance is the lowest calculated metric of a path to reach the destination.

  • EGP

    Exterior Gateway Protocol. A routing protocol used to exchange routing information among two routers in a network of autonomous systems. An EGP protocol maintains routes between autonomous systems.


    Enhanced Interior Gateway Routing Protocol. A Cisco proprietary interior gateway protocol that integrates the capabilities of distance-vector protocols with the capabilities of link-state protocols: It chooses a routing path based on vectors and converges rapidly by using change-based updates. EIGRP supports CIDR and VLSM and uses the DUAL algorithm to perform distributed shortest-path routing with fast convergence while remaining loop free at every instant.

    EIGRP supports the following routed network communication protocols: IPv4, AppleTalk, and Novell NetWare. The IPv4 implementation of EIGRP is the one that Network Protocol Manager for EIGRP discovers and monitors.

  • EIGRP domain

    A collection of connected EIGRP routers and EIGRP networks and is synonymous to an autonomous system. An EIGRP domain provides full connectivity to all hosts within it.

    All EIGRP routers that belong to the same EIGRP domain are assigned the same autonomous system number. (Also, see EIGRP process number.)

  • EIGRP neighbors

    EIGRP routers that share a common segment become neighbors on that segment. Neighbors, also known as peers, are elected through the Hello protocol. Two-way communication exists between each pair of neighbors.

  • EIGRP network

    Interconnected routers, on the same IP subnet, that are running EIGRP services.

  • EIGRP peers

    See “EIGRP neighbors” on page 55 .

  • EIGRP process number

    Synonymous with autonomous system number. EIGRP routers that have the same process number belong to the same EIGRP domain.

  • EIGRP router

    A router that is running an EIGRP service.

  • EIGRP service

    An instance of the EIGRP routing protocol that is running in memory.

  • EIGRP session

    A link between EIGRP neighbors. An EIGRP session, also known as an EIGRP neighbor adjacency, is the next step after EIGRP routers become neighbors. Once the routers establish a session by using the Hello protocol, they exchange routing information and store the neighbor’s address and interface as an entry in their respective neighbor tables. A router’s neighbor table contains information about all the routers known to be directly connected neighbors.

  • IGP

    Interior Gateway Protocol. A routing protocol used to calculate routes and exchange routing information among routers within an autonomous system.

  • IOS

    Internet Operating System. The operating system used by Cisco routers.

  • MTTR

    Mean Time To Repair. The average time to repair/restore a failed machine or system to an acceptable operating condition.

  • MTU

    Maximum transmission unit. A setting that controls the maximum IP packet size that a PC will send.

  • Multi-access segment

    A network supporting three or more routers. A network segment is part of an Ethernet or other network on which all message traffic is common to all nodes, that is, a message is broadcast from one node on the segment and received by all others on the segment.

  • NBMA

    Non-Broadcast Multiple Access. A network without broadcast capabilities, but where all interfaces on the network are fully meshed (connected); for example, a fully meshed Frame Relay cloud.

  • SSH (or SSH1)

    Secure Shell. A command line interface used to securely log in to and access commands on a remote computer. SSH provides strong authentication and secure encrypted communications over an unsecure channel.

  • SSH2

    Secure Shell version 2. A version of SSH that is a more secure, efficient, and portable than SSH1. Note that the SSH1 and SSH2 protocols are not compatible with one another.

  • Telnet

    A command line interface used to log in to and access commands on a remote computer. Telnet does not use secure (encrypted) transmissions.

  • TLV

    Type, length, and value. TLVs are blocks of specific routing-related information in EIGRP packets.

  • VLSM

    Variable length subnet mask. The extension of standard IP classful masks (A, B, and C) to include subnets. Routing protocols (such as EIGRP, IS-IS, and OSPF) that carry the subnet mask within their route updates are able to recognize subnets and forward datagrams within networks that have been subnetted.