D-Link xStack Reference Manual page 209

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xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
Router A can now build its routing table using the network addresses and costs calculated in building the above
shortest path tree.
Areas and Border Routers
OSPF link-state updates are forwarded to other routers by flooding to all routers on the network. OSPF uses the
concept of areas to define where on the network routers that need to receive particular link-state updates are
located. This helps ensure that routing updates are not flooded throughout the entire network and will reduce the
amount of bandwidth consumed by updating the various router's routing tables.
Areas establish boundaries beyond which link-state updates do not need to be flooded. So the exchange of link-
state updates and the calculation of the shortest path tree are limited to the area that the router is connected to.
Routers that have connections to more than one area are called Border Routers (BR). The Border Routers have
the responsibility of distributing necessary routing information and changes between areas.
Areas are specific to the router interface. A router that has all of its interfaces in the same area is called an Internal
Router. A router that has interfaces in multiple areas is called a Border Router. Routers that act as gateways to
other networks (possibly using other routing protocols) are called Autonomous System Border Routers (ASBRs).
Link-State Packets
There are a number of different types of link-state packets, four of which are illustrated below:
Router Link-State Updates - These describe a router's links to destinations within an area.
Summary Link-State Updates - Issued by Border Routers and describe links to networks outside the area but
within the Autonomous System (AS).
Network Link-State Updates - Issued by multi-access areas that have more than one attached router. One
router is elected as the Designated Router (DR) and this router issues the network link-state updates describing
every router on the segment.
External Link-State Updates - Issued by an Autonomous System Border Router and describes routes to
destinations outside the AS or a default route to the outside AS.
The format of these link-state updates is described in more detail below.
Router link-state updates are flooded to all routers in the current area. These updates describe the destinations
reachable through all of the router's interfaces.
Summary link-state updates are generated by Border Routers to distribute routing information about other networks
within the AS. Normally, all Summary link-state updates are forwarded to the backbone (area 0) and are then
forwarded to all other areas in the network. Border Routers also have the responsibility of distributing routing
information from the Autonomous System Border Router in order for routers in the network to get and maintain
routes to other Autonomous Systems.
Network link-state updates are generated by a router elected as the Designated Router on a multi-access segment
(with more than one attached router). These updates describe all of the routers on the segment and their network
connections.
External link-state updates carry routing information to networks outside the Autonomous System. The
Autonomous System Border Router is responsible for generating and distributing these updates.
OSPF Authentication
OSPF packets can be authenticated as coming from trusted routers by the use of predefined passwords. The
default for routers is to use no authentication.
There are two other authentication methods: Simple Password Authentication (key) and Message Digest
authentication (MD-5).
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