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Introduction

To provide support for IP route, expand and modify IS-IS, so that IS-IS can simultaneously apply in TCP / IP and OSI environments. Integrated IS-IS (Integrated IS-IS or DUAL IS-IS). The IS-IS protocol that is now referred to is the integrated IS-IS protocol, mainly used for Member Networks and bearer networks.

IS-IS is within the internal gateway routing protocol for the inside of the autonomous system. IS-IS is a link status protocol that is very similar to the OSPF protocol in the TCP / IP network, and routing is performed using the shortest path priority algorithm.

ISO network and network layer address of the IP network are different. The three-layer address of the IP network is a common IPv4 address or IPv6 address, and the IS-IS protocol refers to NSAP (Network Service Access Point, Network Service Access Point) to describe the network address structure of the ISO model.

The network running the IS-IS protocol contains the end system, an intermediate system, a region (Area), and Routing Domain. A router is Intermediate System (IS), and one host is End System (ES). The protocol run between the host and the router is called the ES-IS, and the protocol running between the router and the router is called IS-IS. The area is the subdivision unit of routing domains, and IS-IS allows the entire routing domain into multiple regions, and IS-IS is used to provide routing in the routing domain or in a region.

IS-IS area

In order to support large-scale routing networks, IS-IS uses two stratified structures in the routing domain. A large routing domain is divided into one or more areas. And define three roles of the router: Level-1, Level-2, Level-1-2. Routing in the area through the Level-1 router management, the route between the regions is managed through the Level-2 router. Below, the following three types of router roles:

  • Level-1 router is responsible for the route in the area, it is only associated with Level-1 and Level-1-2 routers belonging to the same area. To form a neighbor relationship, maintain a link status database of a Level-1, which contains routing information in this area, forwards to the nearest Level-1-2 router. is-is

  • Level-2 router is responsible for the route between the zone, which can form a neighbor relationship with the LEVEL-2 and Level-1-2 routers in the same area or other area, and maintain a level- 2 Link Status Database, the link status database contains routing information between the area. All Level-2 routers and Level-1-2 routers constitute a routing domain backbone, responsible for communication between different regions, the Level-2 router in the routing domain must be physically continuous to ensure continuity of the backbone network.

  • The router belonging to Level-1 and Level-2 is called the Level-1-2 router, which can be formed with the LEVEL-1 and Level-1-2 routers of the same area. Level-1 neighbor relationship can also form a neighbor relationship of Level-2 in the LEVEL-2 and Level-1-2 routers of the same area or other region. The Level-1 router must be connected to other areas through the Level-1-2 router. Level-1-2 router maintains two link status databases, LEVEL-1 link status databases for regional routes, link status databases of Level-2 are used for inter-area routes.

Each router can only belong to one area, and the area boundary is on the link.

IS-IS PDU

is communicating between the router, IS-IS is used by ISO defined protocol data unit (PDU). The PDU type used in IS-IS is: IS-IS Hello PDU (IIH PDU), link status PDU (LSP), full serial number packet (CSNP), partial number data packet (PSNP).

  • IIH PDU is similar to Hello packets in the OSPF protocol, responsible for forming a neighbor relationship between the router, discovering a new neighbor, detecting whether there is a neighbor exit.

  • LSP is similar to LSA in the OSPF protocol, used to describe all link status information in the router.

  • CSNP contains summary information for each LSP in the network. When the router receives a CSNP, it compares the CSNP with its link status database if When the router lost a LSP exists in the CSNP, it sends a multicast PSNP to the LSP you need from other routers in the network.

  • PSNP is used to confirm the received LSP in point-to-point link; in point-to-point link and broadcast links are used to request the latest version or lost LSP.

IS-IS collects and exchanges information with the surrounding routers to calculate the IS-IS routing entry.

IS-IS features

  • Maintain a link status database and use the SPF algorithm to calculate the best path;

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    Establish and maintain neighbor relationships with Hello packages;

  • use zone to construct a topological structure of two levels;

  • You can use routing between the regions to reduce the burden on the router;

  • support VLSM and CIDR;

  • Manage and control the flood spread of the network by electing IS (DIS) by electing the IS (DIS);

  • has an authentication function;

  • IS-IS uses COST as a metric;

  • fast convergence;

  • is suitable for large networks .

    • address type

    IS-IS has two address types:

    Network Service Access Point (NSAP) - NSAP Address Used to identify a network layer service, each service corresponds to an NSAP address.

    Network Entity Title (NET) - NET address is used to identify a network layer entity or process, not a service.

    Each device may not only contain one address, but NET should be unique and the NSAP system ID portion must also be unique in each system.

    When IS-IS is used as IGP, most companies are in the simplest NSAP format, which make up as follows:

    1. Regional address, at least one byte, The following two parts are composed.

    ---- AFI is set to 49: indicating that AFI is local management, so the company has the right to assign each address.

    ----- Regional Mark (ID): Yes the byte located behind the AFI.

    The router in the same area must use the same area address, this address defines the area. Regional address is used for L2 routing. ES can only identify IS and ES with the same area address in the same subnet.

    2. System ID: The 6-byte system ID is required in the Cisco router, and the system ID must be unique, and the MAC address of the router is usually used as the system ID. However, integrated IS-IS uses IP addresses as part of the system ID. In the entire AS, the system ID should be unique, so that when the device is moved to other regions, the L1 or L2 collision will not cause.

    3.nsel: For the router is always 0

    protocol structure

    IS-IS PDU header structure:

    8 16bit < / P>

    intradomain routing protocol discriminator length indicator

    version / protocol id extension ID length

    RRR PDU Type Version

    Reserved Maximum Area Addresses

    intradomain routing protocol discriminator - The network layer protocol identifier assigned to the protocol.

    length indicator - fixed header (OCTET).

    Version / Protocol ID Extension - value is 1.

    ID length - refers to the NSAP address and the ID field of the NET used in the route domain.

    r - reserved bit.

    PDU Type - PDU type. Bit 6, bit 7 and bit 8 as a reserved bit.

    Version - value is 1.

    Maximum Area Addresses - The area address number licensed by the intermediate system area.

    IS-IS NSAP format:

    AFI IDI Contents Assigned by Authority Identified In IDi Field

    IDP - initial domain part (Initial Domain Part).

    AFI - Permission format identifier (Authority and FormatiTifier, 1 byte). Provide information and content such as IDi and DSP fields.

    IDi - The initial domain identifier (Initial Domain Identifier can become long).

    DSP - Domain Specific Parts

    HO-DSP - High Order Domain Spectial Part.

    area address (Area Address, which can be growing).

    ID - System ID (1-8 bytes).

    NSEL - selector (1 byte, its function is equivalent to the port number in the Internet of Agreement).

    Related protocol OSPF, ES-IS, CLNP, IDRP, CONP

    Organizational Source IS-IS Definition In ISO Document 10589, the RFC 1629 of the IETF document is reforming.

    Agreement Certification

    IS-IS Certification is limited to the express textport, Cisco's iOS supports 3 level certifications:

    1. Neighbor authentication - mutual The connected router interface must be configured with the same password, and the respective authentication must be configured for the L1 and L2 type neighbor relationships. The password of the L1 neighbor authentication and the password of the L2 neighbor can be different. Neighbor authentication is configured by the "ISIS Password".

    2. Regional Authentication - Each router in the area must perform authentication, and must use the same password. Regional authentication is configured by command "Area-Password".

    3. Domain Certification-Domain Each L2 and L1 / L2 type router must perform authentication and must use the same password. Domain authentication is configured by command "Domain-Password".

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