cnp3-ebook/principles/network — English @ Weblate

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187 ../../principles/linkstate.rst:94

	English Flooding is illustrated in the figure below. By exchanging HELLO messages, each router learns its direct neighbors. For example, router `E` learns that it is directly connected to routers `D`, `B` and `C`. Its first LSP has sequence number `0` and contains the directed links `E->D`, `E->B` and `E->C`. Router `E` sends its LSP on all its links and routers `D`, `B` and `C` insert the LSP in their LSDB and forward it over their other links. 441/10000 · 441
188 ../../principles/linkstate.rst:157

	English Flooding allows LSPs to be distributed to all routers inside the network without relying on routing tables. In the example above, the LSP sent by router `E` is likely to be sent twice on some links in the network. For example, routers `B` and `C` receive `E`'s LSP at almost the same time and forward it over the `B-C` link. To avoid sending the same LSP twice on each link, a possible solution is to slightly change the pseudo-code above so that a router waits for some random time before forwarding a LSP on each link. The drawback of this solution is that the delay to flood an LSP to all routers in the network increases. In practice, routers immediately flood the LSPs that contain new information (e.g. addition or removal of a link) and delay the flooding of refresh LSPs (i.e. LSPs that contain exactly the same information as the previous LSP originating from this router) [FFEB2005]_. 894/10000 · 894
189 ../../principles/linkstate.rst:159

	English To ensure that all routers receive all LSPs, even when there are transmissions errors, link state routing protocols use `reliable flooding`. With `reliable flooding`, routers use acknowledgments and if necessary retransmissions to ensure that all link state packets are successfully transferred to each neighboring router. Thanks to reliable flooding, all routers store in their LSDB the most recent LSP sent by each router in the network. By combining the received LSPs with its own LSP, each router can build a graph that represents the entire network topology. 563/10000 · 563
190 ../../principles/linkstate.rst:239

	English Static or dynamic link metrics ? 32/10000 · 32
191 ../../principles/linkstate.rst:241

	English As link state packets are flooded regularly, routers are able to measure the quality (e.g. delay or load) of their links and adjust the metric of each link according to its current quality. Such dynamic adjustments were included in the ARPANET routing protocol [MRR1979]_ . However, experience showed that it was difficult to tune the dynamic adjustments and ensure that no forwarding loops occur in the network [KZ1989]_. Today's link state routing protocols use metrics that are manually configured on the routers and are only changed by the network operators or network management tools [FRT2002]_. 601/10000 · 601
192 ../../principles/linkstate.rst:245

	English When a link fails, the two routers attached to the link detect the failure by the absence of HELLO messages received during the last :math:`k \times N` seconds. Once a router has detected the failure of one of its local links, it generates and floods a new LSP that no longer contains the failed link. This new LSP replaces the previous LSP in the network. In practice, the two routers attached to a link do not detect this failure exactly at the same time. During this period, some links may be announced in only one direction. This is illustrated in the figure below. Router `E` has detected the failure of link `E-B` and flooded a new LSP, but router `B` has not yet detected this failure. 692/10000 · 692
193 ../../principles/linkstate.rst:334

	English When a link is reported in the LSP of only one of the attached routers, routers consider the link as having failed and they remove it from the directed graph that they compute from their LSDB. This is called the `two-way connectivity check`. This check allows link failures to be quickly flooded as a single LSP is sufficient to announce such bad news. However, when a link comes up, it can only be used once the two attached routers have sent their LSPs. The `two-way connectivity check` also allows for dealing with router failures. When a router fails, all its links fail by definition. These failures are reported in the LSPs sent by the neighbors of the failed router. The failed router does not, of course, send a new LSP to announce its failure. However, in the graph that represents the network, this failed router appears as a node that only has outgoing edges. Thanks to the `two-way connectivity check`, this failed router cannot be considered as a transit router to reach any destination since no outgoing edge is attached to it. 1041/10000 · 1041
194 ../../principles/linkstate.rst:336

	English When a router has failed, its LSP must be removed from the LSDB of all routers [#foverload]_. This can be done by using the `age` field that is included in each LSP. The `age` field is used to bound the maximum lifetime of a link state packet in the network. When a router generates a LSP, it sets its lifetime (usually measured in seconds) in the `age` field. All routers regularly decrement the `age` of the LSPs in their LSDB and a LSP is discarded once its `age` reaches `0`. Thanks to the `age` field, the LSP from a failed router does not remain in the LSDBs forever. 573/10000 · 573
195 ../../principles/linkstate.rst:338

	English To compute its forwarding table, each router computes the spanning tree rooted at itself by using Dijkstra's shortest path algorithm [Dijkstra1959]_. The forwarding table can be derived automatically from the spanning as shown in the figure below. 247/10000 · 247
196 ../../principles/linkstate.rst:380

	English It should be noted that link state routing assumes that all routers in the network have enough memory to store the entire LSDB. The routers that do not have enough memory to store the entire LSDB cannot participate in link state routing. Some link state routing protocols allow routers to report that they do not have enough memory and must be removed from the graph by the other routers in the network, but this is outside the scope of this e-book. 449/10000 · 449
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