NOTE:
By default, the router ID is typically the lowest-numbered IP address or the lowest-numbered
(user-configured) loopback interface configured on the device.
If multiple networks exist in the same OSPF area, the recommended approach is to ensure that each
network uses a different router as its DR. Otherwise, if a router is a DR for more than one network,
latency in the router could increase because of the increased traffic load resulting from multiple DR
assignments.
When only one router on an OSPF network claims the DR role despite neighboring routers with higher priorities or
router IDs, this router remains the DR. This is also true for BDRs.
The DR and BDR election process is performed when one of the following events occurs:
• Interface is in a waiting state and the wait time expires
• Interface is in a waiting state and a hello packet is received that addresses the BDR
• Change in the neighbor state occurs, such as:
◦ Neighbor state transitions from 2 or higher
◦ Communication to a neighbor is lost
◦ Neighbor declares itself to be the DR or BDR for the first time
OSPF area types
OSPF is built upon a hierarchy of network areas. All areas for a given OSPF domain reside in the same AS. An
AS is defined as a number of contiguous networks, all of which share the same interior gateway routing protocol.
An AS can be divided into multiple areas. Each area represents a collection of contiguous networks and hosts,
and the topology of a given area is not known by the internal routers in any other area. Areas define the
boundaries to which types 1 and 2 LSAs are broadcast, which limits the amount of LSA flooding that occurs within
the AS and also helps to control the size of the LSDBs maintained in OSPF routers. An area is represented in
OSPF by either an IP address or a number. Area types include:
• Backbone
• Normal
• Not-so-stubby (NSSA)
• Stub
Chapter 11 Open Shortest Path First Protocol (OSPF)
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