Because OSPF routing is
dependent on the status of a link between two routers, neighbor
routers must "recognize" each other on the network before
they can share information. Neighbor routers are routers that share
a common data link, as shown in the figure. This process is done
using the Hello protocol (part of the OSPF suite of protocols). This
protocol enables neighbor routers to establish virtual
point-to-point links called adjacencies and to ensure bi-directional
communication between neighbors before exchanging link-state
information.
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Lab
Activity |
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In this lab,
you will learn how to enable OSPF routing protocol in Area
0 only. |
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Hello packets are sent periodically out of each interface using
IP multicast addresses. Multicast addresses are used to send packets
to a group of devices, rather than a single router (unicast) or the
entire network (broadcast). The information contained in a Hello
packet is as follows:
- Router ID --- Used to identify a router to OSPF. The highest
IP address on an active physical or loopback interface is used as
the Router ID. This identification is important in establishing
neighbor relationships and coordinating messages between OSPF
routers on the network. Also, the Router ID is used to break ties
during the DR and BDR election processes if the priority values
are equal. (DR and BDR are discussed later.)

- Hello interval --- Specifies the frequency in seconds that a
router sends out Hello messages (the default is 10 seconds on
multiaccess networks), as shown in the Figure. The Hello interval
must be the same on neighboring routers.

- Dead interval --- The dead interval is the time in seconds
that a router waits to hear from a neighboring router before
declaring the neighbor down (four times the Hello interval by
default). The Dead interval must be the same on neighboring routers.

- This Hello process also provides quicker detection of failed
routers because Hellos are exchanged every 10 seconds. Routers
expect to hear from their neighbors every 10 seconds. If a router
is silent for 40 seconds, its neighbors believe it is down. Note:
This is true if you assume default configuration values.
- Neighbors --- The neighbors to which adjacencies have been
created. (At the initial startup point, this field is empty because
the first router has not received Hello messages from other
routers.)

- Area-ID --- The common identification number for an area. To
communicate, two routers must share a common network segment, have
interfaces configured to belong to the same area on that segment,
and share the same subnet address and mask. Routers from the same
area will each have the same link-state database information.

- Router Priority --- A value that indicates the priority of
this router when selecting a Designated Router (DR) and Backup
Designated Router (BDR). The default priority is 1 and can be
configured to a higher number to ensure a specified router becomes
the DR.

- DR and BDR --- The IP addresses of the DR and BDR for the
specific network, if known.

- Authentication password --- If authentication is enabled, two
routers must exchange the same password. Authentication is optional.

- Stub area flag --- Indicates the presence of a stub area. A stub
area is a special area that has only one router that connects to
another area. Stub areas are discussed in the next chapter. Two routers
must agree on the stub area flag in the Hello packets.

Neighbor relationships differ based on the types of connection
that exist between the routers , as follows:
- Broadcast multiaccess --- In a typical broadcast LAN
environment such as Ethernet or Token Ring, the OSPF routers
communicate with the DR by using multicast address 224.0.0.6 (aka
All DR Routers).. The DR sends this learned information back to
the OSPF routers by using multicast address 224.0.0.5 (aka All SPF
Routers). Neighbor relationships are formed based on a dynamic
learning process using multicast Hellos (explained earlier).
- Point-to-point --- Across a serial connection, for example,
neighbor relationships are formed only between two routers on each
point-to-point link. Because there are only two routers on this type
of connection, no DR or BDR is needed or used.
- Nonbroadcast multiaccess (NBMA) --- Across a standard Frame
Relay or ATM network, there is no broadcast capability. As such, all
OSPF messages in a nonbroadcast environment are sent as unicast. As
such, neighbor relationships often require additional configuration.
However, OSPF routers on an NBMA network do elect a DR and a BDR.
The exchange process, when all routers come up on the network at
the same time, uses the Hello protocol. This process is detailed in the following steps:
- Router A is enabled on the LAN, but OSPF is in the down
state because it has not exchanged information with any other
router. Router A sends a Hello packet containing information about
itself to the network. This information includes Router A's Router
ID.
- All OSPF routers with the IP multicast address enabled
receive the Hello packet from Router A and store the information
about Router A locally. This is the Init state.
- All routers that received the Hello packet send a unicast
reply Hello packet to Router A with their corresponding
information. A unicast packet is a packet addressed to one
specific device on the network. The information that the
responding routers send includes details about all neighboring
routers, including Router A.
- When Router A receives these reply packets, it adds all the
routers that had its (Router A's) Router ID in their packet to its
own database. This step is referred to as the two-way
state. At this point, all routers that have learned about each
other establish bi-directional communication.
- Next, neighbor routers select the DR and BDR. The DR and BDR
election process is described in the "Step 2" next
section. This process must occur before routers can begin
exchanging the complete link-state information (information about
the network topology and routing devices). Link-state exchanges
are discussed in the "Step 3" section to follow.
- Neighboring routers then determine the master/slave
relationship between themselves (e.g. who will initiate sending
the link-state advertisements?), exchange LSAs that describe each
router's link state database, and establish full adjacency.
- Periodically (10 seconds by default), the routers within a
network exchange Hello packets to ensure communication is still
working. These Hello "keepalive" updates include the DR
router ID and the list of routers whose Hello packets have been
received by the router sending the Hello.
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