Figure 6- 99. 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 0, 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).
•
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.
105
Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Router A
Router B
Router C
128.213.0.0
192.213.11.0
222.211.10.0
0
10
10
5
10
