rfc1195.ps

RFC 的详细文档！

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0.7 0 32 (Usually, all nodes in an area have the same area address. However, sometimes an area might have) W
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(multiple addresses. Motivations for allowing this are:) h
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16.6 0 32 (It might be desirable to change the address of an area. The most graceful way of changing an) W
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56.2 0 32 (area from having address A to having address B is to first allow it to have both addresses A) W
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84.3 0 32 (and B, and then after all nodes in the area have been modified to recognize both addresses,) W
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(then one by one the nodes can be modified to ) h
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(forget) h
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( address A.) h
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94.6 0 32 (It might be desirable to merge areas A and B into one area. The method for accomplishing) W
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137.7 0 32 (this is to, one by one, add knowledge of address B into the A partition, and similarly add) W
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(knowledge of address A into the B partition.) h
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58.2 0 32 (It might be desirable to partition an area C into two areas, A and B \(where ) W
58.2 0 32 (\026) W
58.2 0 32 (A) W
58.2 0 32 (\027) W
58.2 0 32 ( might equal) W
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42.6 0 32 (\026) W
42.6 0 32 (C) W
42.6 0 32 (\027) W
42.6 0 32 (, in which case this example becomes one of removing a portion of an area\). This would) W
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212.1 0 32 (be accomplished by first introducing knowledge of address A into the appropriate nodes) W
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86.7 0 32 (\(those destined to become area A\), and knowledge of address B into the appropriate nodes,) W
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(and then one by one removing knowledge of address C. ) h
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104.4 0 32 (Since OSI addressing explicitly identifies the area, it is very easy for level 1 routers to identify) W
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(packets going to destinations outside of their area, which need to be forwarded to level 2 routers.) h
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(In IS\255IS, there are two types of routers:) h
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121.6 0 32 (Level 1 intermediate systems ) W
121.6 0 32 (\002) W
121.6 0 32 ( these nodes route based on the ID portion of the ISO ad\255) W
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236.7 0 32 (dress. They route within an area. They recognize, based on the destination address in a) W
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25.3 0 32 (packet, whether the destination is within the area. If so, they route towards the destination. If) W
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(not, they route to the nearest level 2 router.) h
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165.4 0 32 (Level 2 intermediate systems ) W
165.4 0 32 (\002) W
165.4 0 32 ( these nodes route based on the area address \(i.e., on the) W
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159.4 0 32 (combination of [IDP, HO\255DSP]\). They route towards areas, without regard to the internal) W
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(structure of an area. A level 2 IS may also be a level 1 IS in one area.) h
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120.0 0 32 (A level 1 router will have the area portion of its address manually configured. It will refuse to) W
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18.5 0 32 (become a neighbor with a node whose area addresses do not overlap its area addresses. However,) W
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35.5 0 32 (if level 1 router has area addresses A, B, and C, and a neighbor has area addresses B and D, then) W
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(the level 1 router will accept the other node as a neighbor.) h
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20.3 0 32 (A level 2 router will accept another level 2 router as a neighbor, regardless of area address. How\255) W
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161.2 0 32 (ever, if the area addresses do not overlap, the link would be considered by both routers to be) W
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145.2 0 32 (\026) W
145.2 0 32 (level 2 only) W
145.2 0 32 (\027) W
145.2 0 32 (, and only level 2 LSPs would flow on the link. External links \(to other routing) W
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(domains\) must be from level 2 routers.) h
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129.6 0 32 (IS\255IS provides an optional partition repair function. In the unlikely case that a level 1 area be\255) W
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2.5 0 32 (come partitioned, this function, if implemented, allows the partition to be repaired via use of level) W
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(2 routes.) h
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96.3 0 32 (IS\255IS requires that the set of level 2 routers be connected. Should the level 2 backbone become) W
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(partitioned, there is no provision for use of level 1 links to repair a level 2 partition.) h
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(December, 1990) h
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133.6 0 32 (In unusual cases, a single level 2 router may lose connectivity to the level 2 backbone. In this) W
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63.3 0 32 (case the level 2 router will indicate in its level 1 LSPs that it is not ) W
63.3 0 32 (\026) W
63.3 0 32 (attached) W
63.3 0 32 (\027) W
63.3 0 32 (, thereby allowing) W
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89.9 0 32 (level 1 routers in the area to route traffic for outside of the domain to a different level 2 router.) W
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73.4 0 32 (Level 1 routers therefore route traffic to destinations outside of their area only to level 2 routers) W
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(which indicate in their level 1 LSPs that they are ) h
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(attached) h
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(.) h
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23.4 0 32 (An end system may autoconfigure the area portion of its address by extracting the area portion of) W
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2.4 0 32 (a neighboring router's address. If this is the case, then an endnode will always accept a router as a) W
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100.4 0 32 (neighbor. Since the standard does not specify that the end system MUST autoconfigure its area) W
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202.6 0 32 (address, an end system may be configured with an area address. In this case the end system) W
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(would ignore router neighbors with non\255matching area addresses.) h
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55.2 0 32 (Special treatment is necessary for broadcast subnetworks, such as LANs. This solves two sets of) W
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87.3 0 32 (issues: \(i\) In the absence of special treatment, each router on the subnetwork would announce a) W
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66.9 0 32 (link to every other router on the subnetwork, resulting in n\255squared links reported; \(ii\) Again, in) W
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75.5 0 32 (the absence of special treatment, each router on the LAN would report the same identical list of) W
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(end systems on the LAN, resulting in substantial duplication.) h
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0.9 0 32 (These problems are avoided by use of a ) W
0.9 0 32 (\026) W
0.9 0 32 (pseudonode) W
0.9 0 32 (\027) W
0.9 0 32 (, which represents the LAN. Each router on) W
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91.1 0 32 (the LAN reports that it has a link to the pseudonode \(rather than reporting a link to every other) W
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2.8 0 32 (router on the LAN\). One of the routers on the LAN is elected ) W
2.8 0 32 (\026) W
2.8 0 32 (designated router) W
2.8 0 32 (\027) W
2.8 0 32 (. The designated) W
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40.9 0 32 (router then sends out an LSP on behalf of the pseudonode, reporting links to all of the routers on) W
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69.0 0 32 (the LAN. This reduces the potential n\255squared links to n links. In addition, only the pseudonode) W
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123.8 0 32 (LSP includes the list of end systems on the LAN, thereby eliminating the potential duplication) W
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(\(for further information on designated routers and pseudonodes, see [1]\).) h
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86.1 0 32 (The IS\255IS provides for optional Quality of Service \(QOS\) routing, based on throughput \(the de\255) W
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121.9 0 32 (fault metric\), delay, expense, or residual error probability. This is described in greater detail in) W
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(section 3.5, and in [1].) h
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(1.3 ) h
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(Overview of the Integrated IS\255IS) h
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17.8 0 32 (The integrated IS\255IS allows a single routing protocol to be used to route both IP and OSI packets.) W
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133.6 0 32 (This implies that the same two\255level hierarchy will be used for both IP and OSI routing. Each) W
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104.5 0 32 (area will be specified to be either IP\255only \(only IP traffic can be routed in that particular area\),) W
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133.8 0 32 (OSI\255only \(only OSI traffic can be routed in that area\), or dual \(both IP and OSI traffic can be) W
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(routed in the area\). ) h
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61.5 0 32 (This proposal does not allow for partial overlap of OSI and IP areas. For example, if one area is) W
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74.6 0 32 (OSI\255only, and another area is IP\255only, then it is not permissible to have some routers be in both) W
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16.9 0 32 (areas. Similarly, a single backbone is used for the routing domain. There is no provision for inde\255) W
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(pendent OSI and IP backbones.) h
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74.1 0 32 (Similarly, within an IP\255only or dual area, the amount of knowledge maintained by routers about) W
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44.0 0 32 (specific IP destinations will be as similar as possible as for OSI. For example, IP\255capable level 1) W
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21.7 0 32 (routers will maintain the topology within the area, and will be able to route directly to IP destina\255) W
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149.1 0 32 (tions within the area. However, IP\255capable level 1 routers will not maintain information about) W