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../../protocols/routing.rst:76
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Area border routers : A router that is attached to several areas.
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../../protocols/routing.rst:78
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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.
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../../protocols/routing.rst:84
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OSPF areas
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../../protocols/routing.rst:86
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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.
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../../protocols/routing.rst:92
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Hierarchical routing with OSPF
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../../protocols/routing.rst:94
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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:
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../../protocols/routing.rst:96
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`RB` advertises `2001:db8:1234::/48` at a distance of `2` and `2001:db8:5678::/48` at a distance of `3`
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../../protocols/routing.rst:97
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`RC` advertises `2001:db8:5678::/48` at a distance of `2` and `2001:db8:1234::/48` at a distance of `3`
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../../protocols/routing.rst:99
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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.
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../../protocols/routing.rst:101
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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 :
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../../protocols/routing.rst:103
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`2001:db8:aaaa:0000::/64` at a distance of `1` and `2001:db8:aaaa:0001::/64` at a distance of `2` from `RA`
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../../protocols/routing.rst:104
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`2001:db8:aaaa:0000::/63` at a distance of `2` from `RA`
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../../protocols/routing.rst:106
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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.
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../../protocols/routing.rst:110
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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.
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../../protocols/routing.rst:130
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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 :
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../../protocols/routing.rst:132
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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.
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../../protocols/routing.rst:133
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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.
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../../protocols/routing.rst:135
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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.
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../../protocols/routing.rst:139
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How to quickly detect a link failure ?
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../../protocols/routing.rst:141
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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]_ :
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