connected. Some routers may try to become adjacent
to all their neighbor routers. Other routers may try to become
adjacent to only one or two neighbor routers. OSPF routers
determine which routers to become adjacent to based on the type
of network they are connected to. Once an adjacency is formed
between neighbors, link-state information is exchanged. OSPF
interfaces recognize three types of networks:
- Broadcast multi-access, such as Ethernet
- Point-to-point networks
- Nonbroadcast multi-access
(NBMA), such as Frame Relay
A fourth type,
point-to-multipoint, can be configured on an interface by an
administrator. In a multiaccess network, the number of routers
that will be connected in advance is unknown. In point-to-point
networks, only two routers can be connected. In a broadcast
multi-access network segment, many routers may be connected. If
every router had to establish full adjacency with every other
router and exchange link-state information with every neighbor,
there would be too much overhead. If there are 5 routers, 10
adjacency relationships would be needed and 10 link states
sent. If there are 10 routers then 45 adjacencies would be
needed. In general, for n routers, n*(n-1)/2 adjacencies would
need to be formed. The solution to this overhead is to hold an
election for a designated router (DR). This router becomes
adjacent to all other routers in the broadcast segment. All
other routers on the segment send their link-state information
to the DR. The DR in turn acts as the spokesperson for the
segment. Using the example numbers above, only 5 and 10 sets of
link states need be sent respectively. The DR sends link-state
information to all other routers on the segment using the
multicast address of 224.0.0.5 for all OSPF routers. Despite
the gain in efficiency that electing a DR provides, there is a
disadvantage. The DR represents a single point of failure. A
second router is elected as a backup designated router (BDR) to
take over the duties of the DR if it should fail. To ensure
that both the DR and the BDR see the link states all routers
send on the segment, the multicast address for all designated
routers, 224.0.0.6, is used. On point-to-point networks only
two nodes exist and no DR or BDR is elected. Both routers
become fully adjacent with each other. Interactive Media
Activity Drag and Drop: OSPF Network Types When the student
has completed this activity, the student will be able to
identify the different OSPF network types. Web Links
Areas and Border Routers http://www.cisco.com/warp/public/
104/2.html#3.0
Content 2.2 Single Area OSPF
Concepts 2.2.6 OSPF Hello protocol When a router starts
an OSPF routing process on an interface, it sends a hello
packet and continues to send hellos at regular intervals. The
rules that govern the exchange of OSPF hello packets are called
the Hello protocol. At Layer 3 of the OSI model, the hello
packets are addressed to the multicast address 224.0.0.5. This
address is “all OSPF routers”. OSPF routers use hello packets
to initiate new adjacencies and to ensure that neighbor routers
are still functioning. Hellos are sent every 10 seconds by
default on broadcast multi-access and point-to-point networks.
On interfaces that connect to NBMA networks, such as Frame
Relay, the default time is 30 seconds. On multi-access networks
the Hello protocol elects a designated router (DR) and a backup
designated router (BDR). Although the hello packet is small, it
consists of the OSPF packet header. For the hello packet the
type field is set to 1. The hello packet carries information
that all neighbors must agree upon before an adjacency is
formed, and link-state information is exchanged.
Interactive Media Activity Drag and Drop: OSPF Packet
Header When the student has completed this activity, the
student will be able to identify the different fields in an
OSPF packet header. Web Links Neighbors
http://www.cisco.com/warp/public/ 104/2.html#9.0
Content
2.2 Single Area OSPF Concepts 2.2.7 Steps in the
operation of OSPF OSPF routers send Hello packets on OSPF
enabled interfaces. If all parameters in the OSPF Hello packets
are agreed upon, the routers become neighbors. On multi-access
networks, the routers elect a DR and BDR. On these networks
other routers become adjacent to the DR. Adjacent routers go
through a sequence of states. Adjacent routers must be in the
full state before routing tables are created and traffic
routed. Each router sends link-state advertisements (LSA) in
link-state update (LSU) packets. These LSAs describe all of the
routers links. Each router that receives an LSA from its
neighbor records the LSA in the link-state database. This
process is repeated for all routers in the OSPF network. When
the databases are complete, each router uses the SPF algorithm
to calculate a loop free logical topology to every known
network. The shortest path with the lowest cost is used in
building this topology, therefore the best route is selected.
Routing information is now maintained. When there is a change
in a link state, routers use a flooding process to notify other
routers on the network about the change. The Hello protocol
dead interval provides a simple mechanism for determining that
an adjacent neighbor is down. - Interactive Media
Activity Drag and Drop: OSPF State Flowchart When the
student has completed this activity, the student will be able
to identify the different OSPF neighbor states. Web
Links OSPF Packets http://www.juniper.net/techpubs/
software/junos50/ swconfig50-routing/ html/ospf-overview6.html
Content 2.3 Single Area OSPF Configuration
2.3.1 Configuring OSPF routing process OSPF routing
uses the concept of areas. Each router contains a complete
database of link-states in a specific area. An area in the OSPF
network, it may be assigned any number from 0 to 65,535.
However a single area is assigned the number 0 and is known as
area 0. In multi-area OSPF networks, all areas are required to
connect to area 0. Area 0 is also called the backbone area.OSPF
configuration requires that the configuration be enabled on the
router with network addresses and area information. Network
addresses are configured with a wildcard mask and not a subnet
mask. The wildcard mask represents the links or host addresses
that can be present in this segment. Area IDs can be written as
a whole number or dotted decimal notation. To enable OSPF
routing, use the global configuration command syntax:
Router(config)#router ospf process-id The process
ID is a number that is used to identify an OSPF routing process
on the router. Multiple OSPF processes can be started on the
same router. The number can be any value between 1 and 65,535.
Most network administrators keep the same process ID throughout
an autonomous system, but this is not a requirement. It is
rarely necessary to run more than one OSPF process on a router.
IP networks are advertised as follows in OSPF:
Router(config-router)#network address wildcard-mask
area area-id Each network must be identified
with the area to which it belongs. The network address can be a
whole network, a subnet, or the address of the interface. The
wildcard mask represents the set of host addresses that the
segment supports. This is different than a subnet mask, which
is used when configuring IP addresses on interfaces. Lab
Activity Lab Exercise: Configuring the OSPF Routing
Process This lab is to setup an IP addressing scheme for OSPF
area 0 and configure and verify OSPF routing. Lab
Activity e-Lab Activity: Configuring OSPF In this lab, the
students will configure and verify OSPF routing. Web
Links Enabling OSPF on the Router
http://www.cisco.com/warp/public/ 104/2.html#5.0
Content
2.3 Single Area OSPF Configuration 2.3.2
Configuring OSPF loopback address and router priority When the
OSPF process starts, the Cisco IOS uses the highest local
active IP address as its OSPF router ID. If there is no active
interface, the OSPF process will not start. If the active