Section 3.1. Comparative Terminology
3.1. Comparative TerminologyThis first section has nothing to do with messages, but it has to go somewhere. So here you are. OSPF is a child of the IETF, whereas IS-IS is born of the ISO, and they frequently use different terms to describe the same things. So before getting into messages, I want to spend a short time listing these redundant terms. The most obvious terminology difference is between the IETF term router (or, that relic of earlier times, gateway) and the ISO term intermediate system (IS). Similarly, an IETF host is in ISO terms an end system (ES), and ES-IS is a protocol spoken between end systems and intermediate systems (hosts and routers). ES-IS is only used for CLNP, never for IP, so it is not mentioned again in this book. It is relevant here as a point of contrast to IS-IS, a protocol spoken between intermediate systems: a routing protocol. With few exceptions, I use the term router in this book to refer to both OSPF and IS-IS nodes, and I always use the term host, rather than ES. A router identifies itself by a router ID, whereas an intermediate system identifies itself by a system ID, often abbreviated to sys ID. Functionally a router ID and a sys ID are the same thing, but I use both terms to differentiate OSPF and IS-IS routers. IS-IS defines a term subnetwork point of attachment (SNPA), which is a point at which subnetwork services are provided by a device to a subnetwork. You can think of an SNPA as a logical (not physical) interface connected to a data link, and represented by a data link address such as a MAC address. IETF terminology defines a network layer data unit as a packet or, less frequently, datagram. At the data link layer, a data entity is a frame. ISO uses the more flexible protocol data unit (PDU) to describe data units at all layers: subnetwork PDUs, network PDUs, and so on. I like the term PDU, and often use it even in the context of OSPF. The basic unit of link state routing information that we called, in the previous chapter, a link state announcement is in OSPF a link state advertisement (LSA) and in IS-IS a link state PDU (LSP). Although they serve the same fundamental purpose of populating the link state database, the difference between LSAs and LSPs is one of the more distinct differences between the two protocols. LSAs and LSPs are introduced in Section 3.5 in this chapter, and are detailed in Chapter 5. The IETF concept of autonomous system (AS) is, in ISO terms, a routing domain. I much prefer the second term. These days, an AS has a specific meaning in BGP networks, to differentiate one area of autonomous administrative control from another, and as entities interconnected by EBGP. Within a single AS, multiple IGPs can be running. In contrast, a routing domain is always the scope of a single set of routers speaking the same routing protocol to each other, unbroken by any other routing protocol. In this book, I use routing domain to describe either an OSPF or an IS-IS domain except where specific IETF terminology is required (such as the OSPF term autonomous system boundary router). Finally, there are some differences in area terminology. Both OSPF and IS-IS use a two-level area hierarchy, as introduced in the preceding chapter. OSPF calls the higher-level area a backbone area, or simply area 0 based on the area ID that always signifies the OSPF backbone. Lower-level OSPF areas are designated by the unwieldy term nonbackbone area. IS-IS identifies the higher-level area as level 2 (L2), and the lower-level areas as level 1 (L1) areas. IS-IS does not actually refer to the L2 area as an areainstead, it is called the L2 subdomain. Chapter 7 discusses the nuances of the differences between an area and a subdomain. Table 3.1 summarizes the most common terminology differences between OSPF and ISIS. With the basic jargon explained, we are ready to begin looking at message types.
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EAN: 2147483647
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