D-Link DGS-3620-28TC-SI Product Manual - Page 176

xStack® DGS-3620 Series Managed Switch Web UI Reference Guide is higher than the route through Router C. This higher-cost route will not be included in the Router A's shortest path tree. The resulting tree will look like this: 128.213.0.0 Router A 0 10 10 Router B Router C 5 10 192.213.11.0 222.211.10.0 Figure 5-20 Constructing a Shortest Path Tree - Completed Note that this shortest path tree is only from the viewpoint of Router A. The cost of the link from Router B to Router A, for instance is not important to constructing Router A's shortest path tree, but is very important when Router B is constructing its shortest path tree. Note also that directly connected networks are reached at a cost of zero, while other networks are reached at the cost calculated in the shortest path tree. 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 linkstate 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: 1. Router Link-State Updates - These describe a router's links to destinations within an area. 2. Summary Link-State Updates - Issued by Border Routers and describe links to networks outside the area but within the Autonomous System (AS). 166