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	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.
	In some networks, there are several dozens of paths towards a given destination. Some routers, due to hardware limitations, cannot install more than 8 or 16 paths in their forwarding table. In this case, a subset of the computed paths is installed in the forwarding table.
	Internal router : A router whose directly connected networks belong to the area
	In this formula, `N` is the number of outgoing interfaces on the equal cost paths towards the packet's destination. Various hash functions are possible, including CRC, checksum or MD5 :rfc:`2991`. Since the hash function is computed over the four-tuple, the same hash value will be computed for all packets belonging to the same flow. This prevents reordering due to load balancing inside the network. Most routers support this kind of load-balancing today [ACO+2006]_.
	In this section, we briefly describe the key features of the two main intradomain unicast routing protocols : RIP and OSPF. The basic principles of distance vector and link-state routing have been presented earlier.
	Intradomain routing
	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:
	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]_.
	:math:`hash(NextHeader,IP_{src},IP_{dst},Port_{src},Port_{dst}) \pmod{N}`
	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]_ :
	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 :
	Organisation of a small Internet
	OSPF
	OSPF areas
	OSPF can support `virtual links` to connect routers together that belong to the same area but are not directly connected. However, this goes beyond this introduction to OSPF.
	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`:
	`RB` advertises `2001:db8:1234::/48` at a distance of `2` and `2001:db8:5678::/48` at a distance of `3`
	`RC` advertises `2001:db8:5678::/48` at a distance of `2` and `2001:db8:1234::/48` at a distance of `3`
	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.
	RIP