cnp3-ebook/protocols/routing — English @ Weblate

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19 ../../protocols/routing.rst:65

	English OSPF 4/10000 · 4
20 ../../protocols/routing.rst:67

	English Link-state routing protocols are used in IP networks. Open Shortest Path First (OSPF), defined in :rfc:`2328`, is the link state routing protocol that has been standardized by the IETF. The last version of OSPF, which supports IPv6, is defined in :rfc:`5340`. OSPF is frequently used in enterprise networks and in some ISP networks. However, ISP networks often use the IS-IS link-state routing protocol [ISO10589]_ , which was developed for the ISO CLNP protocol but was adapted to be used in IP :rfc:`1195` networks before the finalization of the standardization of OSPF. A detailed analysis of ISIS and OSPF may be found in [BMO2006]_ and [Perlman2000]_. Additional information about OSPF may be found in [Moy1998]_. 719/10000 · 719
21 ../../protocols/routing.rst:71

	English Compared to the basics of link-state routing protocols that we discussed in section :ref:`linkstate`, there are some particularities of OSPF that are worth discussing. First, in a large network, flooding the information about all routers and links to thousands of routers or more may be costly as each router needs to store all the information about the entire network. A better approach would be to introduce hierarchical routing. Hierarchical routing divides the network into regions. All the routers inside a region have detailed information about the topology of the region but only learn aggregated information about the topology of the other regions and their interconnections. OSPF supports a restricted variant of hierarchical routing. In OSPF's terminology, a region is called an `area`. 796/10000 · 796
22 ../../protocols/routing.rst:73

	English OSPF imposes restrictions on how a network can be divided into areas. An area is a set of routers and links that are grouped together. Usually, the topology of an area is chosen so that a packet sent by one router inside the area can reach any other router in the area without leaving the area [#fvirtual]_ . An OSPF area contains two types of routers :rfc:`2328`: 364/10000 · 364
23 ../../protocols/routing.rst:75

	English Internal router : A router whose directly connected networks belong to the area 79/10000 · 79
24 ../../protocols/routing.rst:76

	English Area border routers : A router that is attached to several areas. 65/10000 · 65
25 ../../protocols/routing.rst:78

	English For example, the network shown in the figure below has been divided into three areas : `area 0`, containing routers `RA`, `RB`, `RC` and `RD`, `area 1`, containing routers `R1`, `R3`, `R4`, `R5` and `RA`, and `area 2` containing `R7`, `R8`, `R9`, `R10`, `RB` and `RC`. OSPF areas are identified by a 32 bit integer, which is sometimes represented as an IP address. Among the OSPF areas, `area 0`, also called the `backbone area`, has a special role. The backbone area groups all the area border routers (routers `RA`, `RB` and `RC` in the figure below) and the routers that are directly connected to the backbone routers but do not belong to another area (router `RD` in the figure below). An important restriction imposed by OSPF is that the path between two routers that belong to two different areas (e.g. `R1` and `R8` in the figure below) must pass through the backbone area. 880/10000 · 880
26 ../../protocols/routing.rst:84

	English OSPF areas 10/10000 · 10
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

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