cnp3-ebook/protocols/routing — English @ Weblate

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27 ../../protocols/routing.rst:86

	English Inside each non-backbone area, routers distribute the topology of the area by exchanging link state packets with the other routers in the area. The internal routers do not know the topology of other areas, but each router knows how to reach the backbone area. Inside an area, the routers only exchange link-state packets for all destinations that are reachable inside the area. In OSPF, the inter-area routing is done by exchanging distance vectors. This is illustrated by the network topology shown below. 506/10000 · 506
28 ../../protocols/routing.rst:92

	English Hierarchical routing with OSPF 30/10000 · 30
29 ../../protocols/routing.rst:94

	English Let us first consider OSPF routing inside `area 2`. All routers in the area learn a route towards `2001:db8:1234::/48` and `2001:db8:5678::/48`. The two area border routers, `RB` and `RC`, create network summary advertisements. Assuming that all links have a unit link metric, these would be: 292/10000 · 292
30 ../../protocols/routing.rst:96

	English `RB` advertises `2001:db8:1234::/48` at a distance of `2` and `2001:db8:5678::/48` at a distance of `3` 103/10000 · 103
31 ../../protocols/routing.rst:97

	English `RC` advertises `2001:db8:5678::/48` at a distance of `2` and `2001:db8:1234::/48` at a distance of `3` 103/10000 · 103
32 ../../protocols/routing.rst:99

	English These summary advertisements are flooded through the backbone area attached to routers `RB` and `RC`. In its routing table, router `RA` selects the summary advertised by `RB` to reach `2001:db8:1234::/48` and the summary advertised by `RC` to reach `2001:db8:5678::/48`. Inside `area 1`, router `RA` advertises a summary indicating that `2001:db8:1234::/48` and `2001:db8:5678::/48` are both at a distance of `3` from itself. 425/10000 · 425
33 ../../protocols/routing.rst:101

	English On the other hand, consider the prefixes `2001:db8:aaaa:0000::/64` and `2001:db8:aaaa:0001::/64` that are inside `area 1`. Router `RA` is the only area border router that is attached to this area. This router can create two different network summary advertisements : 266/10000 · 266
34 ../../protocols/routing.rst:103

	English `2001:db8:aaaa:0000::/64` at a distance of `1` and `2001:db8:aaaa:0001::/64` at a distance of `2` from `RA` 107/10000 · 107
35 ../../protocols/routing.rst:104

	English `2001:db8:aaaa:0000::/63` at a distance of `2` from `RA` 56/10000 · 56
36 ../../protocols/routing.rst:106

	English The first summary advertisement provides precise information about the distance used to reach each prefix. However, all routers in the network have to maintain a route towards `2001:db8:aaaa:0000::/64` and a route towards `2001:db8:aaaa:0001::/64` that are both via router `RA`. The second advertisement would improve the scalability of OSPF by reducing the number of routes that are advertised across area boundaries. However, in practice this requires manual configuration on the border routers. 497/10000 · 497
37 ../../protocols/routing.rst:110

	English The second OSPF particularity that is worth discussing is the support of Local Area Networks (LAN). As shown in the example below, several routers may be attached to the same LAN. 179/10000 · 179
38 ../../protocols/routing.rst:130

	English A first solution to support such a LAN with a link-state routing protocol would be to consider that a LAN is equivalent to a full-mesh of point-to-point links as if each router can directly reach any other router on the LAN. However, this approach has two important drawbacks : 277/10000 · 277
39 ../../protocols/routing.rst:132

	English Each router must exchange HELLOs and link state packets with all the other routers on the LAN. This increases the number of OSPF packets that are sent and processed by each router. 180/10000 · 180
40 ../../protocols/routing.rst:133

	English Remote routers, when looking at the topology distributed by OSPF, consider that there is a full-mesh of links between all the LAN routers. Such a full-mesh implies a lot of redundancy in case of failure, while in practice the entire LAN may completely fail. In case of a failure of the entire LAN, all routers need to detect the failures and flood link state packets before the LAN is completely removed from the OSPF topology by remote routers. 445/10000 · 445
41 ../../protocols/routing.rst:135

	English To better represent LANs and reduce the number of OSPF packets that are exchanged, OSPF handles LAN differently. When OSPF routers boot on a LAN, they elect [#felection]_ one of them as the `Designated Router (DR)` :rfc:`2328`. The `DR` router `represents` the local area network, and advertises the LAN's subnet. Furthermore, LAN routers only exchange HELLO packets with the `DR`. Thanks to the utilization of a `DR`, the topology of the LAN appears as a set of point-to-point links connected to the `DR` router. 513/10000 · 513
42 ../../protocols/routing.rst:139

	English How to quickly detect a link failure ? 38/10000 · 38
43 ../../protocols/routing.rst:141

	English Network operators expect an OSPF network to be able to quickly recover from link or router failures [VPD2004]_. In an OSPF network, the recovery after a failure is performed in three steps [FFEB2005]_ : 202/10000 · 202
44 ../../protocols/routing.rst:143

	English the routers that are adjacent to the failure detect it quickly. The default solution is to rely on the regular exchange of HELLO packets. However, the interval between successive HELLOs is often set to 10 seconds... Setting the HELLO timer down to a few milliseconds is difficult as HELLO packets are created and processed by the main CPU of the routers and these routers cannot easily generate and process a HELLO packet every millisecond on each of their interfaces. A better solution is to use a dedicated failure detection protocol such as the Bidirectional Forwarding Detection (BFD) protocol defined in [KW2009]_ that can be implemented directly on the router interfaces. Another solution to be able to detect the failure is to instrument the physical and the datalink layer so that they can interrupt the router when a link fails. Unfortunately, such a solution cannot be used on all types of physical and datalink layers. 929/10000 · 929
45 ../../protocols/routing.rst:144

	English the routers that have detected the failure flood their updated link state packets in the network 96/10000 · 96
46 ../../protocols/routing.rst:145

	English all routers update their routing table 38/10000 · 38

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