Specifications

64 Implementing IBM System Networking 10Gb Ethernet Switches
OSPF router types
As shown in Figure 2-9 OSPF uses the following types of routing devices:
򐂰 Internal Router (IR): A router that has all of its interfaces within the same area. IRs
maintain LSDBs identical to the LSDBs of other routing devices within the local area.
򐂰 Area Border Router (ABR): A router that has interfaces in multiple areas. ABRs maintain
one LSDB for each connected area and disseminate routing information between areas.
򐂰 Autonomous System Boundary Router (ASBR): A router that acts as a gateway between
the OSPF domain and non-OSPF domains, such as RIP, BGP, and static routes.
Figure 2-9 OSPF router types
Neighbors and adjacencies
In areas with two or more routing devices, neighbors and adjacencies are formed. Neighbors
are routing devices that maintain information about each others’ health. To establish neighbor
relationships, routing devices periodically send hello packets on each of their interfaces. All
routing devices that share a common network segment, appear in the same area, and have
the same health parameters (
hello and dead intervals), authentication parameters, area
number, and area stub-flag respond to each other’s hello packets and become neighbors.
Neighbors continue to send periodic hello packets to advertise their health to neighbors. In
turn, they listen to hello packets to determine the health of their neighbors and to establish
contact with new neighbors. On broadcast networks (like Ethernet), the hello process is used
for electing one of the neighbors as the area’s Designated Router (DR) and one as the area’s
Backup Designated Router (BDR). The DR is next to all other neighbors and acts as the
central contact for database exchanges. Each neighbor sends its database information to the
DR, which relays the information to the other neighbors.
The BDR is next to all other neighbors (including the DR). Each neighbor sends its database
information to the BDR as with the DR, but the BDR merely stores this data and does not
distribute it. If the DR fails, the BDR takes over the task of distributing database information to
the other neighbors.
Link State Database
OSPF is a link-state routing protocol. A link represents an interface (or routable path) from
the routing device. By establishing an adjacency with the DR, each routing device in an OSPF
area maintains an identical Link-State Database (LSDB) describing the network topology for
its area.