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

Begin by examining Interior gateway protocol and exterior gateway protocol links and IS-IS areas within an IS-IS domain:

Figure 1. Interior gateway protocol and exterior gateway protocol links
Figure 2. IS-IS areas within an IS-IS domain

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.

IS-IS 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 IS-IS domain.)

  • 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.

  • CLNP

    Connectionless Network Protocol. An ISO network-layer datagram protocol, which is defined in RFC 1561, that provides fundamentally the same underlying service to a transport layer (Layer 4) as IP. CLNP may be used between network entities in end systems (hosts), between network entities in intermediate systems (routers), or between network entities in end and intermediate systems.

    CLNP uses OSI NSAP addresses and NETs to identify network-layer services and devices.

  • CLNS

    Connectionless Network Service. An ISO network-layer service that does not require a circuit to be established before data is transmitted. CLNS divides messages into packets and routes each packet to its destination independently of any other packet.

    Three network-layer protocols work together to deliver CLNS:

    • CLNP

    • ES-IS

    • IS-IS

  • DIS

    Designated Intermediate System. An IS-IS router elected on a broadcast (multiaccess) network segment to flood routing updates. The DIS creates a logical router called a pseudonode, and each router on the broadcast network forms an adjacency to the pseudonode and to each other. On behalf of the pseudonode, the DIS generates one advertisement for the entire broadcast network (LAN). In addition, the DIS ensures that all routers on the LAN maintain synchronized databases—maintain identical views of the area topology—by broadcasting periodic link-state database updates to the routers.

    Separate DISs are elected for L1 and L2 routing. Election of the DIS is based on the highest interface priority. In case of a tie, the router with the highest SPNA address (MAC address, for example) for the interface is elected the DIS. No backup DIS is elected (exists) for L1 or L2 routing.

  • 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.

  • ES

    End System. In ISO terminology, a workstation or network host that has limited routing capability.

  • ES-IS

    End System to Intermediate System. An OSI protocol that defines how end systems (hosts) and intermediate systems (routers) discover each other in a pure ISO environment, such as the environment implemented in Digital’s DECnet Phase V networking architecture. The host-to-router connections are made by forming adjacencies between the ES and the IS.

    In an IP environment, instead of participating in the ES-IS protocol, IP hosts rely on the address resolution protocol (ARP) for Layer 3 to Layer 2 address resolution to determine the Layer 2 addresses of LAN-connected hosts and the IP default gateway. Similarly, IP routers that are running the IS-IS routing protocol use ARP to locate IP hosts on directly connected segments.

  • 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.

  • IS

    Intermediate System. In ISO terminology, an intermediate system is a router: A network device that has extensive packet-forwarding capabilities and whose role is to relay data between user applications that are running on distantly located end systems (hosts). I ntermediate refers to the capabilities of routers as intermediate forwarding or relay devices.

  • IS-IS

    Intermediate System to Intermediate System. An OSI interior gateway protocol, which is defined in ISO 10589, that is designed to distribute routing information within an autonomous system. IS-IS is a link-state hierarchical routing protocol in which intermediate systems (routers) exchange routing information based on a single metric to determine network topology.

    IS-IS supports CIDR and VLSM and uses a two-level hierarchy, L1 and L2, for controlling distribution of routing information within IS-IS areas (L1 routing) and between IS-IS areas (L2 routing). IS-IS uses the Dijkstra algorithm for route calculations.

    Although originally developed to route CLNP packets in ISO CLNP networks, IS-IS is now more often used to route IP packets in IP networks. The IPv4 implementation of IS-IS, called Integrated or Dual IS-IS and defined in draft-ietf-isis-wg-mib-16.txt, is the implementation that Network Protocol Manager for IS-IS discovers and monitors.

  • IS-IS adjacency

    A link between IS-IS neighbors. An IS-IS adjacency, also known as an IS-IS neighbor adjacency or an IS-IS neighbor relationship, is the next step after IS-IS routers become neighbors. The type of adjacency formed depends on the parameters exchanged in the IS-IS Hello packets.

    Routers in the same area must be able to form at least an L1 adjacency, regardless of the type of interconnecting links: Point-to-point or broadcast. On Cisco routers, the default mode of operation for routers in the same IS-IS area is to form both L1 and L2 adjacencies. Routers that belong to different areas can form only L2 adjacencies.

    For an IS-IS adjacency on a broadcast link, each of the two routers forming the adjacency runs the DIS election process to determine whether it is eligible to be an L1 or L2 DIS on the broadcast network (LAN).

  • IS-IS area

    A group of joining IS-IS networks and attached end systems (hosts) specified by a network administrator or manager. IS-IS areas provide a manageable hierarchy within an IS-IS domain.


    All IS-IS areas are stubs, which means that no routes external to the IS-IS domain are advertised into the areas. Routing from a stub area to the outside world is based on a default route.

  • IS-IS domain

    A collection of connected IS-IS areas and is synonymous to an autonomous system. An IS-IS domain provides full connectivity to all end systems (hosts) within it.

    An IS-IS domain makes use of two-level hierarchical routing to control distribution of routing information within and between its IS-IS areas. L1 routing is routing within an IS-IS area, and L2 routing is routing between IS-IS areas.


    IS-IS area boundaries are on the links between routers (unlike the OSPF routing protocol, where the area boundaries are within routers), which means that area membership is assigned to a router as a whole.

  • IS-IS interface

    A link between an IS-IS router and a network. The state of an IS-IS interface is a description of the interface and its relationship to its neighboring routers. One or more router adjacencies may develop over an interface.

  • IS-IS neighbors

    IS-IS routers that share a common segment become neighbors on that segment. Neighbors are elected through the Hello protocol. Two-way communication exists between each pair of neighbors.

  • IS-IS network

    Interconnected routers, on the same IP subnet, that are running IS-IS services. Only two types of subnets (or links) in an IS-IS network are of practical significance in current applications of the IS-IS protocol: Point-to-point subnets (such as ATM permanent or switched virtual circuits) and broadcast subnets (such as LAN media with broadcast capabilities—Ethernet, for example).

  • IS-IS router

    A router that is running an IS-IS service.

  • IS-IS service

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

  • ISO

    International Organization for Standardization. An international standards organization that develops manufacturing and performance standards for a wide variety of industries.

  • L1 routing

    Level-1 routing. Controls distribution of routing information within an IS-IS area. L1 routing is based on system ID.

    L1 routing can occur between two routers that are configured as L1 routers, between two routers that are configured as L1/L2 routers, or between a router that is configured as an L1 router and a router that is configured as an L1/L2 router. An L1 router maintains a database of all routers within the area and tags L1/L2 routers for use as default routes. An L1/L2 router maintains two separate databases: An L1 database for intra-area routing and an L2 database for inter-area routing. It also advertises a default route into its area.

    Any traffic bound for another area is sent to a router that performs L2 routing.

  • L2 routing

    Level-2 routing. Controls distribution of routing information between IS-IS areas. L2 routing is based on area ID.

    L2 routing can occur between two routers that are configured as L1/L2 routers, between two routers that are configured as L2 routers, or between a router that is configured as an L2 router and a router that is configured as an L1/L2 router. An L2 router maintains a database of all the areas in the IS-IS domain and the closest next-hop L2 or L1/L2 router for each area.

    L2-capable routers compose the IS-IS backbone and can reside in any area. There must be an unbroken chain of L1/L2 or L2 routers in order for the backbone to function.

  • LSP

    Link-State PDU—Portable Data Unit. When a network link changes state (up to down, or vice versa) in an IS-IS network, the change is flooded throughout the network as an LSP, which is a packet in itself without other headers. All the routers note the change and recompute their routes accordingly.

  • 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.

  • NET

    Network Entity Title. A CLNS address that identifies a network-layer entity in an end system (host) or intermediate system (router). NETs are allocated from the same name space as NSAP addresses, and the determination of whether an address is an NSAP address or a NET depends on the context in which the address is interpreted.

    The NET format has three main components:



    <AreaID> = a variable-length area address (1 to 13 bytes).

    <SystemID> = a 6-byte identifier that is unique throughout each IS-IS area (L1)

         and throughout the IS-IS backbone (L2).

    <N-selector> = a 1-byte value that is always 00.


    Because Integrated IS-IS uses a NET address to identify the router, the shortest path first calculations are based on system ID and area ID, not IP subnet.

  • NSAP

    Network Service Access Point. A CLNS address that identifies a network-layer service. An OSI NSAP is the point at which an OSI network-layer (Layer 3) service is made available to a transport layer (Layer 4) entity.

    The OSI NSAP format has four main components:



    <DomainID> = a variable-length domain address (1 to 11 bytes).

    <AreaID> = a variable-length area address (1 to 13 bytes).

    <SystemID> = a 6-byte identifier that is unique throughout each IS-IS area (L1)

         and throughout the IS-IS backbone (L2).

    <N-selector> = any 1-byte value other than 00.

  • OSI

    Open Systems Interconnection. A standard description or reference model for how messages should be transmitted between any two points in a telecommunication network.

  • Pseudonode

    In IS-IS, a logical node emulating a broadcast link. The pseudonode role is played by an elected DIS.

  • SNPA

    Subnetwork point of attachment. An interface that attaches to a subnet.

    The SNPA address refers to a data-link address, and would be the Media Access Control (MAC) address for an Ethernet network, the X.25 address for an X.25 network, or the Data Link Connection Identifier (DLCI) for a Frame Relay network.

  • 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 IS-IS 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.