Intermediate System to Intermediate System Protocol (IS-IS)


IS-IS uses type-length-variable (TLV) parameters to carry information in Link-State PDUs (LSPs). Each IS-IS LSP consists of a variable-length header to which TLVs are appended in order to extend IS-IS for IP routing. The TLV field consists of one octet of type (T), one octet of length (L), and “L” octets of Value (V). They are included in all of the IS-IS Packet types. For a complete breakdown of the LSP, see LSP structure on page 420.

In IS-IS, TLVs are used to determine route-leaking and authentication, and are also used for IPv4 and IPv6 awareness and reachability.

  • To determine which TLVs are responsible for route-leaking, see Default routing on page 423.
  • To determine which TLVs are responsible for authentication, see Authentication on page 424. For a complete list of reserved TLV codepoints, refer to RFC 3359.


LSP structure

It is difficult to fully understand a routing protocol without knowing what information is carried in its packets. Knowing how routers exchange each type of information will help you better understand the IS-IS protocol and will allow you to configure your network more appropriatey.

This section provides information on the contents of the IS-IS LSP. LSPs describe the network topology and can include IP routes and checksums.


IS-IS routing protocol utilizes ISO network addressing to identify network interfaces. The addresses are known as Network Service Access Points (NSAPs). In general, IS-IS routers consist of only one NSAP, whereas IP addressing requires one IP address per interface.

In IS-IS, the NSAP address is translated into a Network Entity Title (NET), which is the same as the NSAP but can differentiate end systems by way of a byte called the nselector (NSEL). In order for adjacencies to form in IS-IS, the NSEL must necessarily be set to zero, to indicate “this system”. The total NET can be anywhere between 8 and 20 bytes long due to the support for variable length area addressing.

The following diagram identifies the individual parts of the NSAP, with explanations below.


NSAP and NET example

AFI — The Authority and Format Identifier (AFI) specifies the format of the addressing family used. IS-IS is designed to carry routing information for several different protocols. Each entry has an address family identifier that identifies the globally unique Interdomain Part (IDP). For example, 49 is the AFI for private addresses, whereas 47 is the AFI for international organizations.

IDI — The Initial Domain Identifier (IDI) identifies the routing domain within an interconnected network. The length of the IDI is typically determined by the AFI. If you are using an AFI of 49, you do not need to specify an IDI, since the network is private.

HODSP — The High Order Domain-Specific Part (HODSP) identifies the unique address within a specific routing domain. Together, the AFI, IDI, and HODSP define the area address. All of the nodes within an area must have the same area address.

System ID — The System ID represents the 6-8 byte router identifier. The ID could be Media Access Control (MAC) format, as in the example above, or a static length IP address expressed in binary-coded decimal (BCD) format.

NSEL — The nselector (NSEL), as previously described, identifies the network layer transport service and must always be set to zero for IS-IS NETs.

Parts and terminology of IS-IS

Before you can understand how IS-IS functions, you need to understand some of the main concepts and parts of IS-IS.

This section includes:

  • DIS election and pseudonode LSP
  • Packet types
  • Default routing
  • Timer options
  • Authentication

DIS election and pseudonode LSP

In IS-IS routing protocol, a single router is chosen to be the designated intermediate system (DIS). The election of the DIS is determined automatically and dynamically on the LAN depending on highest interface priority and the subnetwork point of attachment (SNPA). The FortiGate is typically the DIS, and each router in its LAN is an intermediate system (IS).

Unlike OSPF, which elects a designated router (DR) and backup designated router (BDR), the DIS has no backup and determines the election of a new DIS whenever a router is added to the LAN or whenever the current DIS drops. A backup DIS is irrelevant since all of the routers on an IS-IS system are synchronized, and the short Hello interval used by the DIS quickly detects failures and the subsequent replacement of the DIS.

Synchronization of all the nodes in an IS-IS area could prove troublesome when updating the network infrastructure, and would demand ever-increasing resources each time a new router is added (at an exponential scale). For this purpose the DIS creates a pseudonode, which is essentially a virtual, logical node representing

the LAN. The pseudonode requests adjacency status from all the routers in a multi-access network by sending IS- IS Hello (IIH) PDUs to Level 1 and Level 2 routers (where Level 1 routers share the same address as the DIS and Level 2 routers do not). Using a pseudonode to alter the representation of the LAN in the link-state database (LSD) greatly reduces the amount of adjacencies that area routers have to report. In essence, a pseudonode collapses a LAN topology, which allows a more linear scale to link-state advertising.

In order to maintain the database synchronization, the DIS periodically sends complete sequence number packets (CSNPs) to all participating routers.


Packet types

Four general packet types (PDUs) are communicated through IS-IS, appearing at both Level 1 and Level 2. They are described below.


Intermediate System-to-Intermediate System Hello (IIH) PDU — As mentioned previously, the IIH PDU, or Hello packet, detects neighboring routers and indicates to the pseudonode the area’s adjacency mesh. The Hello packet, flooded to the multicast address, contains the system ID of the sending router, the holding time, the circuit type of the interface on which the PDU was sent, the PDU length, the DIS identifier, and the interface priority (used in DIS election). The Hello packet also informs its area routers that it is the DIS.

Hello packets are padded to the maximum IS-IS PDU size of 1492 bytes (the full MTU size) to assist in the detection of transmission errors with large frames or with MTU mismatches between adjacencies.

The DIS typically floods Hello packets to the entire LAN every three seconds.


Link-state PDU (LSP) — The LSP contains information about each router in an area and its connected interfaces. LSPs are refreshed periodically and acknowledged on the network by way of sequence number PDUs. If new LSP information is found, based on the most recent complete sequence number PDU (CSNP), then out-of- date entries in the link-state database (LSDB) are removed and the LSDB is updated.

For a more detailed breakdown of the LSP, see LSP structure on page 420.


Complete sequence number PDU (CSNP) — CSNPs contain a list of all LSPs in the current LSDB. The CSNP informs other area routers of missing or outdated links in the adjacency mesh. The receiving routers then use this information to update their own database to ensure that all area routers converge.

In contrast to Hello packets, CSNPs are sent every ten seconds and only between neighbors. In other words, they are never flooded.


Partial sequence number PDU (PSNP) — PSNPs are used to request and acknowledge LSP information from an adjacency. When a router compares a CSNP with its local database and determines a discrepancy, the router requests an updated LSP using a PSNP. Once received, the router stores the LSP in its local database and responds to the DIS with acknowledgement.

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