User manual

xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
117
Router A
Router B
Router C
128.213.0.0
192.213.11.0
222.211.10.0
0
10
10
5
10
Figure 8- 15. 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 to 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).