This page will explain how routers communicate in an OSPF
network.
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 set of rules that govern the exchange of OSPF Hello packets is called
the Hello protocol. On multi-access networks, the Hello protocol elects a
designated router (DR) and a backup designated router (BDR). The Hello carries
information about which all neighbors must agree to form an adjacency and
exchange link-state information. On multi-access networks the DR and BDR
maintain adjacencies with all other OSPF routers on the network.
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. -
This page concludes this lesson. The next lesson will explain
more about OSPF. The first page will discuss the configuration of OSPF.
Configuring OSPF routing process
2.2.8
This page will teach students how to configure OSPF.
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 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 OSPF routing process be
enabled on the router with network addresses and area information specified. 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
ospfprocess-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.
The Lab Activity will help students configure and verify OSPF
routing.
This next page will teach students how to configure an OSPF loopback
interface. |
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