rfc1195.ps

RFC 的详细文档！

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106.8 0 32 (destinations outside of the area. Just as in normal OSI routing, traffic to destinations outside of) W
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57.8 0 32 (the area will be forwarded to the nearest level 2 router. Since IP routes to subnets, rather than to) W
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150.1 0 32 (specific end systems, IP routers will not need to keep nor distribute lists of IP host identifiers) W
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(\(note that routes to hosts can be announced by using a subnet mask of all ones\).) h
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46.9 0 32 (The IP address structure allows networks to be partitioned into subnets, and allows subnets to be) W
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125.5 0 32 (recursively subdivided into smaller subnets. However, it is undesireable to require any specific) W
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95.7 0 32 (relationship between IP subnet addresses and IS\255IS areas. For example, in many cases, the dual) W
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84.9 0 32 (routers may be installed into existing environments, which already have assigned IP and/or OSI) W
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95.5 0 32 (addresses. In addition, even if IP addresses are not already pre\255assigned, the address limitations) W
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69.6 0 32 (of IP constrain what addresses may be assigned. We therefore will not require any specific rela\255) W
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191.1 0 32 (tionship between IP addresses and the area structure. The IP addresses can be assigned com\255) W
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60.5 0 32 (pletely independently of the OSI addresses and IS\255IS area structure. As will be described in sec\255) W
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251.3 0 32 (tion 3.2 \() W
251.3 0 32 (\026) W
251.3 0 32 (Hierarchical Abbreviation of IP Reachability Information) W
251.3 0 32 (\027) W
251.3 0 32 (\), greater efficiency and) W
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62.8 0 32 (scaling of the routing algorithm can be achieved if there is some correspondence between the IP) W
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(address assignment structure and the area structure. ) h
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34.0 0 32 (Within an area, level 1 routers exchange link state packets which identify the IP addresses reach\255) W
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133.9 0 32 (able by each router. Specifically, zero or more [IP address, subnet mask, metric] combinations) W
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83.7 0 32 (may be included in each Link State Packet. Each level 1 router is manually configured with the) W
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62.6 0 32 ([IP address, subnet mask, metric] combinations which are reachable on each interface. A level 1) W
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(router routes as follows:) h
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(\255) h
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190.2 0 32 (If a specified destination address matches an [IP address, subnet mask, metric] reachable) W
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(within the area, the packet is routed via level 1 routing.) h
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(\255) h
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34.3 0 32 (If a specified destination address does not match any [IP address, subnet mask, metric] com\255) W
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89.8 0 32 (bination listed as reachable within the area, the packet is routed towards the nearest level 2) W
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(router. ) h
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156.4 0 32 (Flexible use of the limited IP address space is important in order to cope with the anticipated) W
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76.4 0 32 (growth of IP environments. Thus an area \(and by implication a routing domain\) may simultane\255) W
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118.2 0 32 (ously make use of a variety of different address masks for different subnets in the area \(or do\255) W
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141.6 0 32 (main\). Generally, if a specified destination address matches more than one [IP address, subnet) W
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27.4 0 32 (mask] pair, the more specific address is the one routed towards \(the one with more "1" bits in the) W
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(mask ) h
(\002) h
( this is known as "best match" routing\).) h
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55.1 0 32 (Level 2 routers include in their level 2 LSPs a complete list of [IP address, subnet mask, metric]) W
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0.5 0 32 (specifying all IP addresses reachable in their area. As described in section 3, this information may) W
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110.1 0 32 (be obtained from a combination of the level 1 LSPs \(obtained from level 1 routers in the same) W
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29.0 0 32 (area\), and/or by manual configuration. In addition, Level 2 routers may report external reachabil\255) W
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33.9 0 32 (ity information, corresponding to addresses which can be reached via routers in other routing do\255) W
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(mains \(autonomous systems\)) h
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96.1 0 32 (Default routes may be announced by use of a subnet mask containing all zeroes. Default routes) W
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65.3 0 32 (should be used with great care, since they can result in ) W
65.3 0 32 (\026) W
65.3 0 32 (black holes) W
65.3 0 32 (\027) W
65.3 0 32 (. Default routes are permit\255) W
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(OSI ISIS for IP and Dual Environments) h
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(December, 1990) h
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48.4 0 32 (ted only at level 2 as external routes \(i.e., included in the "IP External Reachability Information") W
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(field, as explained in sections 3 and 5\). Default routes are not permitted at level 1.) h
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105.1 0 32 (The integrated IS\255IS provides optional Type of Service \(TOS\) routing, through use of the QOS) W
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(feature from IS\255IS.) h
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(1.4 ) h
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(Support of Mixed Routing Domains) h
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(The integrated IS\255IS proposal specifically allows for three types of routing domains:) h
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(\255) h
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(Pure IP) h
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(\255) h
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(Pure OSI) h
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(\255) h
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(Dual) h
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48.3 0 32 (In a pure IP routing domain, all routers must be IP\255capable. IP\255only routers may be freely mixed) W
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11.4 0 32 (with dual routers. Some fields specifically related to OSI operation may be included by dual rout\255) W
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81.7 0 32 (ers, and will be ignored by IP\255only routers. Only IP traffic will be routed in an pure IP domain.) W
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147.4 0 32 (Any OSI traffic may be discarded \(except for the IS\255IS packets necessary for operation of the) W
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(routing protocol\).) h
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98.2 0 32 (In a pure OSI routing domain, all routers must be OSI\255capable. OSI\255only routers may be freely) W
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24.7 0 32 (mixed with dual routers. Some fields specifically related to IP operation may be included by dual) W
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102.3 0 32 (routers, and will be ignored by OSI\255only routers. Only OSI traffic will be routed in a pure OSI) W
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(domain. Any IP traffic may be discarded. ) h
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56.6 0 32 (In a dual routing domain, IP\255only, OSI\255only, and dual routers may be mixed on a per\255area basis.) W
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(Specifically, each area may itself be defined to be pure IP, pure OSI, or dual.) h
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102.2 0 32 (In a pure IP area within a dual domain, IP\255only and dual routers may be freely mixed. Only IP) W
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(traffic can be routed by level 1 routing within a pure\255IP area.) h
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85.9 0 32 (In a pure\255OSI area within a dual domain, OSI\255only and dual routers may be freely mixed. Only) W
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(OSI traffic can be routed by level 1 routing within a pure OSI area.) h
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33.6 0 32 (In a dual area within a dual routing domain only dual routers may be used. Both IP and OSI traf\255) W
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(fic can be routed within a dual area.) h
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89.8 0 32 (Within a dual domain, if both IP and OSI traffic are to be routed between areas then all level 2) W
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(routers must be dual.) h
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/Times-Bold-ISOLatin1 F 1400 o f
(1.5 ) h
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(Advantages of Using Integrated IS\255IS) h
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26.2 0 32 (Use of the integrated IS\255IS protocol, as a single protocol for routing both IP and OSI packets in a) W
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227.3 0 32 (dual environment, has significant advantages over using separate protocols for independently) W
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(routing IP and OSI traffic.) h
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(OSI ISIS for IP and Dual Environments) h
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(December, 1990) h
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183.9 0 32 (An alternative approach is known as ) W
183.9 0 32 (\026) W
183.9 0 32 (Ships In the Night) W
183.9 0 32 (\027) W
183.9 0 32 ( \(S.I.N.\). With the S.I.N. approach,) W
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2.4 0 32 (completely separate routing protocols are used for IP and for OSI. For example, OSPF [5] may be) W
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20.7 0 32 (used for routing IP traffic, and IS\255IS [1] may be used for routing OSI traffic. With S.I.N., the two) W
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77.3 0 32 (routing protocols operate more or less independently. However, dual routers will need to imple\255) W
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243.4 0 32 (ment both routing protocols, and therefore there will be some degree of competition for re\255) W
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(sources.) h
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79.1 0 32 (Note that S.I.N. and the integrated IS\255IS approach are not really completely separate options. In) W
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0.3 0 32 (particular, if the integrated IS\255IS is used within a routing domain for routing of IP and OSI traffic,) W
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(it is still possible to use other independent routing protocols for routing other protocol suites.) h
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171.6 0 32 (In the future, optional extensions to IS\255IS may be defined for routing other common protocol) W
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60.2 0 32 (suites. However, such future options are outside of the scope of this document. This section will) W
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(compare integrated IS\255IS and S.I.N. for routing of IP and OSI only.) h
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95.6 0 32 (A primary advantage of the integrated IS\255IS relates to the network management effort required.) W
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59.2 0 32 (Since the integrated IS\255IS provides a single routing protocol, within a single coordinated routing) W
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166.7 0 32 (domain using a single backbone,, this implies that there is less information to configure. This) W
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(combined with a single coordinated MIB simplifies network management.) h
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20.4 0 32 (Note that the operation of two routing protocols with the S.I.N. approach are not really independ\255) W
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144.2 0 32 (ent, since they must share common resources. However, with the integrated IS\255IS, the interac\255) W
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45.6 0 32 (tions are explicit, whereas with S.I.N., the interactions are implicit. Since the interactions are ex\255) W
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(plicit, again it may be easier to manage and debug dual routers. ) h
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115.8 0 32 (Another advantage of the integrated IS\255IS is that, since it requires only one routing protocol, it) W
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11.2 0 32 (uses fewer resources. In particular, less implementation resources are needed \(since only one pro\255) W
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144.9 0 32 (tocol needs to be implemented\), less CPU and memory resources are used in the router \(since) W
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131.8 0 32 (only one protocol needs to be run\), and less network resources are used \(since only one set of) W
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143.8 0 32 (routing packets need to be transmitted\). Primarily this translates into a financial savings, since) W
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146.3 0 32 (each of these three types of resources cost money. This implies that dual routers based on the) W
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193.4 0 32 (integrated IS\255ginal Finger protocol, this
   murchase and operate than dual routers based on) W
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(S.I.N.) h
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20.4 0 32 (Note that the operation of two routing protocols with the S.I.N. approach are not really independ\255) W
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62.3 0 32 (ent, since they must share common resources. For example, if one routing protocol becomes un\255) W
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121.9 0 32 (stable and starts to use excessive resources, the other protocol is likely to suffer. A bug in one) W
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126.2 0 32 (protocol could crash the other. However, with the integrated IS\255IS, the interactions are explicit) W
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86.2 0 32 (and are defined into the protocol and software interactions. With S.I.N., the interactions are im\255) W
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(plicit.) h
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45.8 0 32 (The use of a single integrated routing protocol similarly reduces the likely frequency of software) W
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76.6 0 32 (upgrades. Specifically, if you have two different routing protocols in your router, then you have) W
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92.6 0 32 (to upgrade the software any time EITHER of the protocols change. If you make use of a single) W
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240.9 0 32 (integrated routing protocol, then software changes are still likely to be needed, but less fre\255) W
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(quently.) h
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38169 -900 M
(December, 1990) h
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161.9 0 32 (Finally, routing protocols