IS-IS Networks and Interfaces

As a CCNA, you are well aware of the ISO OSI networking structure. However, a fundamental distinction must be made at the outset of this chapter. The ISO developed an Open Systems Interconnection (OSI) networking suite that consists of two major components . The first piece is the familiar and often-memorized OSI reference model, which is an abstract network construct represented by a seven-layered model. The second component is the OSI protocol suite, which is a collection of standard networking protocols that includes CLNP and ES-IS, among others, which are covered in this chapter. We are concerned with the OSI Network and Transport layers and their relationship to OSI protocols and services that function at these layers .

OSI Routed Network Protocols and Services

OSI connectionless network services are made possible through the Connectionless Network Protocol (CLNP) and Connectionless Network Service (CLNS), as described in the ISO 8473 standard. CLNP is an OSI Network layer protocol that carries upper-layer data and error messages over connectionless exchanges. CLNP offers the interface between the CLNS and the upper layers. CLNS provides Network layer services to the Transport layer through CLNP. CLNS does not conduct connection setup, maintenance, or termination because the paths are resolved independently for each packet transmitted through a network. Although CLNS does its best to deliver the packet, there is no built-in certainty that data will not be lost, damaged, duplicated , or delivered out of order. CLNS depends on Transport layer protocols to conduct error detection and correction services. This is altogether different from the Connection-Mode Network Service (CMNS).

The Connection-Oriented Network Protocol (CONP) and CMNS provide OSI connection-oriented network services. CONP is an OSI Network layer protocol that transports upper-layer data and error messages over connection-oriented links. CONP is built on the existing X.25 Packet-Layer Protocol (PLP), as defined in ISO 8208. CONP is an OSI protocol that operates at the Network layer and acts as an interface for the upper-layer data and error information of the CMNS. The duties of CMNS are related to the definite formation of communication paths between Transport layer units. These tasks include the setup, maintenance, and termination of connections, as well as individual quality of service (QoS) requests . In a nutshell , for OSI connection-oriented data transfer, CONP is the protocol and CMNS is the service. For OSI connectionless data transfer, CLNP is the protocol and CLNS is the service. End stations do not communicate via the CLNP protocol.

OSI Routing Protocols

OSI breaks up its routing into a multilevel hierarchy to make router design and functionality simpler. Level 0 routing is comparable to the ARP protocol function in IP. The end system “to “intermediate system (ES- IS ) discovery protocol performs routing between hosts and routers to enable the end-to-end exchange of data in the internetwork. Level 1 routing occurs between intermediate systems (ISs), or routers, in the same area. (Every end system exists in a particular area.) A Level 1 IS is concerned only with the path to the nearest Level 2 IS. Level 2 routing happens when Level 1 ISs exchange information between Level 1 areas to create an intradomain core or backbone. In other words, Level 2 routing occurs between different areas within the same routing domain. Level 3 routing is the process of routing between separate domains. Table 6.1 lists Cisco-supported CLNS/CLNP routing protocols.

Table 6.1. OSI CLNS/CLNP Routing Protocols Supported by Cisco

Protocol	Description
ISO-IGRP	A Cisco-proprietary protocol for CLNS based on IGRP that uses distance-vector techniques to exchange routing information.
IS-IS	A link-state protocol utilized in ISO CLNS for dynamic routing of CLNP. The protocol allows routers running as ISs to exchange routing information with other ISs. A Cisco router is a Level 1, Level 2, or Level 1 “2 IS (as an exit point from the area). Integrated IS can propagate IP or CLNS, or both in dual mode.
Static routes (CLNS)	Although not technically a routing protocol, static CLNS routes can be implemented when routing in an OSI CLNS/CLNP environment.

When an end system (ES) needs to send data to another end system, routing can commence as soon as the ES discovers its neighbor through the exchange of hello packets. This process is similar to the way ARP works in IP and is the very nature of Level 0 routing. If the destination end system address is on another network (or subnetwork) in the same area, then the intermediate system ( IS ) knows about the route and sends it along accordingly . This is Level 1 routing. If the destination end system is in a different area, then the Level 1 end system sends the packet to the closest Level 2 IS to be forwarded along until the destination area is reached. This is Level 2 routing. If the packet has to be forwarded to another routing domain, then Level 3 routing takes over. Three primary protocols are used.

Integrated IS-IS Networks

The most current IS-IS specification is ISO 10589, which documents the IS-IS intradomain route exchange functionality. IS-IS was designed to offer Interior Gateway Protocol functionality, whereas the Border Gateway Protocol (BGP) is a widely deployed routing protocol with broad capabilities for interdomain routing. BGP is covered in detail in upcoming Chapters 8, "Configuring Border Gateway Protocol," and 9, "Scaling BGP Networks." Normally, routing protocols support only one OSI Layer 3 protocol. For example, OSPF and IP RIPv2 support the IP-routed protocol. So if you are using routers to concurrently link multiple network layer protocols, they are typically configured with separate routing protocols for each Layer 3 protocol supported (IP RIPv2 and IPX RIP, for example). Integrated IS-IS, however, supports both the ISO CLNP and IP Network layer protocols. The Cisco proprietary Enhanced Interior Gateway Routing Protocol (EIGRP) protocol also simultaneously supports multiple Layer 3 protocols such as IP, IPX, and AppleTalk.

An IS-IS routing domain is a series of interconnected routers (network) that are managed within the same administrative unit. This is also known as an autonomous system of routers or AS. All the routers in the domain run the Integrated IS-IS routing protocol to provide for intradomain exchange of routing information. This Integrated IS-IS network environment can be IP only, ISO CLNP only, or both.

The IS-IS protocol was initially meant to provide support for CLNP only. In RFC 1195, the original IS-IS specification (ISO 10589) is modified to support IP through the designated Integrated IS-IS.

In Table 6.2 you can review some of the most important terms and concepts related to IS-IS. You may want to return to this table throughout this chapter to reinforce your learning.

Table 6.2. Important Terms and Concepts for IS-IS

Term or Concept	Description
Domain	A grouping of interconnected routers (network) that are managed under a shared administrative authority.
Circuit	An IS-IS interface that is uniquely identified with a circuit ID number (8 bytes).
Packet datagram unit (PDU)	Connectionless protocols, such as CLNP and IP, move data in chunks known as packets. In ISO 10589 terminology, packets are called protocol data units.
Link-state PDU (LSP)	This is the link-state packet used to flood information throughout the IS-IS routing domain.
Sequence number packets (SNP)	Control packets used with LSPs to successfully exchange IS-IS routing information.
Intermediate system (IS)	An IS simply represents the OSI term for a router.
Designated intermediate system (DIS)	The one elected router on the local network that is "logically" connected to all other nodes and sends the LSP information on behalf of its LAN. Dijkstra requires a pseudonode for broadcast media to compute shortest path.
End system (ES)	An ES represents the OSI term for a host.
Network service access point (NSAP)	The NSAP is the type of addressing scheme implemented by CLNS and IS-IS for identifying network devices and hosts.
Subnetwork point of attachment (SNPA)	The point at which subnetwork services are offered , usually a MAC address, Frame Relay DLCI, virtual circuit ID for X.25 or ATM, or HDLC.
End System “to “Intermediate xSystem (ES-IS)	A discovery protocol between IS-IS end systems, similar to the IP ARP protocol.
L1L2 IS	An IS, also called a router, that functions at Level 1 and Level 2 and maintains two separate link-state databases.