Information About Implementing Ospf; Ospf Functional Overview - Cisco ASR 9000 Series Configuration Manual

Implementing OSPF
• Configuring authentication (IP Security) is an optional task. If you choose to configure authentication,

Information About Implementing OSPF

To implement OSPF you need to understand the following concepts:

OSPF Functional Overview

OSPF is a routing protocol for IP. It is a link-state protocol, as opposed to a distance-vector protocol. A
link-state protocol makes its routing decisions based on the states of the links that connect source and destination
machines. The state of the link is a description of that interface and its relationship to its neighboring networking
devices. The interface information includes the IP address of the interface, network mask, type of network to
which it is connected, routers connected to that network, and so on. This information is propagated in various
types of link-state advertisements (LSAs).
A router stores the collection of received LSA data in a link-state database. This database includes LSA data
for the links of the router. The contents of the database, when subjected to the Dijkstra algorithm, extract data
to create an OSPF routing table. The difference between the database and the routing table is that the database
contains a complete collection of raw data; the routing table contains a list of shortest paths to known
destinations through specific router interface ports.
OSPF is the IGP of choice because it scales to large networks. It uses areas to partition the network into more
manageable sizes and to introduce hierarchy in the network. A router is attached to one or more areas in a
network. All of the networking devices in an area maintain the same complete database information about the
link states in their area only. They do not know about all link states in the network. The agreement of the
database information among the routers in the area is called convergence.
At the intradomain level, OSPF can import routes learned using Intermediate System-to-Intermediate System
(IS-IS). OSPF routes can also be exported into IS-IS. At the interdomain level, OSPF can import routes learned
using Border Gateway Protocol (BGP). OSPF routes can be exported into BGP.
Unlike Routing Information Protocol (RIP), OSPF does not provide periodic routing updates. On becoming
neighbors, OSPF routers establish an adjacency by exchanging and synchronizing their databases. After that,
only changed routing information is propagated. Every router in an area advertises the costs and states of its
links, sending this information in an LSA. This state information is sent to all OSPF neighbors one hop away.
All the OSPF neighbors, in turn, send the state information unchanged. This flooding process continues until
all devices in the area have the same link-state database.
To determine the best route to a destination, the software sums all of the costs of the links in a route to a
destination. After each router has received routing information from the other networking devices, it runs the
shortest path first (SPF) algorithm to calculate the best path to each destination network in the database.
The networking devices running OSPF detect topological changes in the network, flood link-state updates to
neighbors, and quickly converge on a new view of the topology. Each OSPF router in the network soon has
the same topological view again. OSPF allows multiple equal-cost paths to the same destination. Since all
link-state information is flooded and used in the SPF calculation, multiple equal cost paths can be computed
and used for routing.
On broadcast and nonbroadcast multiaccess (NBMA) networks, the designated router (DR) or backup DR
performs the LSA flooding. On point-to-point networks, flooding simply exits an interface directly to a
neighbor.
OL-30423-03
you must first decide whether to configure plain text or Message Digest 5 (MD5) authentication, and
whether the authentication applies to an entire area or specific interfaces.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
Information About Implementing OSPF
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