We're now going to look at OSPF areas. OSPF has this concept of areas, all networks must have an area 0, which is the "default" area. And areas are used to scale OSPF for really large networks. There are many types of areas, to suit the many different types of infrastructure and topologies. Most midsize networks, today, can easily operate with over 300 routers in a single area. And, in fact, most network operators operate the networks only with area 0. OSPF now supports IPv6, but, in a slightly different way from IS-IS, which we'll see elsewhere in this series. OSPFv2 is developed for IPv4, and has only been designed to carry IPv4 prefixes. For IPv6, OSPF was extended and is now called OSPFv3. And it's only for carrying IPv6 prefixes, there is no support for IPv4 prefixes within OSPFv3. OSPFv3 is documented in RFC5340 and works totally independently from OSPFv2. The configuration concepts and the syntax are very similar. There are subtle differences and improvements, which are really beyond the scope of this presentation. There are two types of links in OSPF. One type is the point-to-point link, where there's only one other router on the link, forming a point-to-point adjacency. The other type of link is the multi-access network, for example, the Ethernet. There's a potential for many other routers to be on this network and with many other adjacencies. OSPF in multi-access networks has optimizations to aid scaling. Two routers are elected to originate the LSAs for the whole multi-access network. These two routers are called the "Designated Router" and "Backup Designated Router" respectively. Other routers on the multi-access network form adjacencies with the DR and the BDR. There is ONE Designated Router per multi-access network. It generates the network link advertisements, assists in the database synchronization, and lets us scale OSPF. And there's also one backup Designated Router. If you look at the diagram in the slide, you see an Ethernet LAN with one Designated Router and one Backup Designated Router. The Designated Router is chosen according to configured priority, and that is set on the interface, on the multi-access network. The highest priority router becomes the DR. The next highest priority becomes the BDR. If there's no priority set, all the priorities are left to the defaults, the priority is determined by the highest router ID. The router ID is a 32-bit integer, so the router ID can be set manually. Otherwise it's derived from the loopback interface IPv4 address. Otherwise, in the absence of a loopback, it's set from the highest IPv4 address on the router. Network operators really don't like this undeterministic way of setting the designated router so they usually, if they're not using a loopback with an IPv4 address, they usually will configure the priority on the interface in question.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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