IS-IS Protocol Overview
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| The IS-IS routing protocol is one of three protocols specified by the International Organiza-tion for Standardization (ISO) to support connectionless network services (CLNS):
ISO CLNS was meant to provide connectionless datagram services for data transmission instead of the conventional connection-oriented services. Unlike connection-oriented services that require end-to-end call establishment to precede any communication between network devices, datagram services allow data to be transmitted in independent chunks , known also as packets, without having to set a predefined path through the network between source and destination before transmission. CLNP, which is very similar to the Internet Protocol (IP), is central to the operation of ISO CLNS. ES-IS and IS-IS are auxiliary protocols that help network nodes (end systems and routers) discover each other and gather routing information, which is used for forwarding packets. For example, the IS-IS protocol provides a dynamic mechanism that allows routers to gather information about various reachable destinations in a network. This information is then processed to determine optimal paths that routers can use for moving data from one end of the network to another. ISO 10589 specifies IS-IS for routing CLNP packets, and RFC 1195 4 provides extensions to ISO 10589 to support routing of IP packets in addition to CLNP packets. Specifically, RFC 1195 defines Integrated (Dual) IS-IS, which allows IS-IS to obtain and also exchange CLNP and IP routing information simultaneously . Despite its dual capabilities, Integrated IS-IS can be used in CLNS-only or IP-only environments. This chapter and the next focuses on use of Integrated IS-IS in IP-only (pure IP) environments. Unlike most routing protocols, which are typically encapsulated in a network layer protocol, IS-IS is itself a network layer protocol and rides over the data link alongside CLNP and IP. Actually, all three ISO protocols that support connectionless networking (CLNP, ES-IS, and IS-IS) are individually network layer protocols. This contrasts with the design of IP-specific routing protocols, such as the Open Shortest Path First (OSPF) and the Border Gateway Proto-cols, which ultimately are encapsulated in IP and operate at a higher layer of the Open System Interconnection (OSI) reference model. Protocol design requires associating a protocol or an application with an identifier for the corresponding layer of operation in the OSI model. The following is a list of network layer protocol identifiers (in binary) for the network layer proto-cols that have been mentioned so far. The hexadecimal equivalent is provided in brackets:
The ISO network layer protocol family is identified at the data link layer by 0xFEFE. IP is identified by 0x0800. CLNP by itself is not relevant to pure IP environments, and only IS-IS essentially is required to support IP routing in such environments. However, the operation of IS-IS is tied intrinsically to certain elements of the ISO CLNS environment, such as ISO addressing, network service access points (NSAP), and the ES-IS protocol. The ES-IS protocol is designed to facilitate communication between CLNS end systems and routers, and it has no relevance to the communication between IP hosts and IP routers. In an IP environment, network devices use IP-associated mechanisms, such as default gateways, the Address Resolution Proto-col (ARP) for IP address-to-data link address resolution, and the Internet Control Message Protocol (ICMP) for network-discovery and control functions. Discussions regarding details of CLNP and ES-IS are beyond the scope of this book, and further discussion is limited to issues of relevance to the operation of the IS-IS protocol. IS-IS Routing ProtocolIS-IS is a link-state protocol designed for intradomain routing. It supports a two-level routing hierarchy:
Routers running the IS-IS protocol form adjacencies with other directly connected IS-IS routers and exchange routing information contained in link-state packets ( LSPs ). Each router collects LSPs into separate Level 1 and Level 2 link-state databases based on their mode of operation (Level 1 only, Level 2 only, or Level 1 ‚ 2). The Level 1 link-state database provides a view of the local area's topology, while the Level 2 link-state database provides a global view of interarea connectivity. The shortest path first (SPF) algorithm (named after Dijkstra) is run separately over Level 1 and Level 2 databases to obtain the best paths to various destinations in the network. IS-IS is one of two popular Interior Gateway Protocols (IGP) used on the large service-provider networks that are interconnected to form the global Internet. The other popular IGP is the Open Shortest Path First (OSPF) protocol. The Border Gateway Protocol (BGP) is used for inter-domain router between network domains (or autonomous systems). Aside from RFC 1195, which allows IS-IS to carry IP routing information, several other enhancements have been proposed for standardization in the IETF. Most prominent of these are Multiprotocol Label Switching Traffic Engineering (MPLS TE) related enhancements 7 . In recent times, interest in the IS-IS protocol has significantly increased, culminating in the reopening of the IS-IS working group in the IETF. Several of the new capabilities proposed in IETF already have been implemented as vendor-specific enhancements, and the effort is directed at standardization and interoperability across different vendor router products. The successful adoption and widespread acceptance of the IS-IS protocol for IP routing is a result of its flexibility for extension, simplicity, and ease of troubleshooting. Troubleshooting of the IS-IS protocol is discussed in the next chapter. This chapter drills into key concepts behind the IS-IS protocol and lays the groundwork for Chapter 11, "Troubleshooting IS-IS." |
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EAN: 2147483647
Pages: 260