sometimes chooses slow routes because it ignores
critical factors such as bandwidth in route determination. OSPF
overcomes these limitations and proves to be a robust and
scalable routing protocol suitable for the networks of today.
OSPF can be used and configured as a single area for small
networks. It can also be used for large networks. OSPF routing
scales to large networks if hierarchical network design
principles are used. Large OSPF networks use a hierarchical
design. Multiple areas connect to a distribution area, area 0,
also called the backbone. This design approach allows for
extensive control of routing updates. Defining areas reduces
routing overhead, speeds up convergence, confines network
instability to an area and improves performance. Web
Links Open Shortest Path First (OSPF)
http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/ospf.htm
Content 2.2
Single Area OSPF Concepts 2.2.2 OSPF terminology
As a link-state protocol, OSPF operates differently from
distance vector routing protocols. Link-state routers identify
neighboring routers and then communicate with the identified
neighbors. OSPF has its own terminology. The new terms are
shown in Figure . Information is gathered from OSPF neighbors
about the status, or links, of each OSPF router. This
information is flooded to all its neighbors. Flooding is a
process that sends information out all ports, with the
exception of the port on which the information was received. An
OSPF router advertises its own link states and passes on
received link states. The routers process the information about
link-states and build a link-state database. Every router in
the OSPF area will have the same link-state database. Every
router has the same information about the state of the links
and the neighbors of every other router. Then each router runs
the SPF algorithm on its own copy of the database. This
calculation determines the best route to a destination. The SPF
algorithm adds up the cost, which is a value that is usually
based on bandwidth. The lowest cost path is added to the
routing table, which is also known as the forwarding database.
OSPF routers record information about their neighbors in the
adjacency database. To reduce the number of exchanges of
routing information among several neighbors on the same
network, OSPF routers elect a Designated Router (DR) and a
Backup Designated Router (BDR) that serve as focal points for
routing information exchange. Interactive Media
Activity Crossword Puzzle: OSPF Terminology When the
student has completed this activity, the student will
understand the different OSPF terminology. Web Links
Open Shortest Path First (OSPF)
http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/ospf.htm
Content 2.2
Single Area OSPF Concepts 2.2.3 Comparing OSPF with
distance vector routing protocols OSPF uses link-state
technology, compared with distance vector technology such as
RIP. Link-state routers maintain a common picture of the
network and exchange link information upon initial discovery or
network changes. Link-state routers do not broadcast their
routing tables periodically as distance vector protocols do.
Therefore, link-state routers use less bandwidth for routing
table maintenance. RIP is appropriate for small networks, and
the best path is based on the lowest number of hops. OSPF is
appropriate for the needs of large scalable internetworks, and
the best path is determined by speed. RIP and other distance
vector protocols use simple algorithms to compute best paths.
The SPF algorithm is complex. Routers implementing distance
vector routing may need less memory and less powerful
processors than those running OSPF. OSPF selects routes based
on cost, which is related to speed. The higher the speed, the
lower the OSPF cost of the link. OSPF selects the fastest
loop-free path from the shortest-path first tree as the best
path in the network. OSPF guarantees loop-free routing.
Distance vector protocols may cause routing loops. If links are
unstable, flooding of link-state information can lead to
unsynchronized link-state advertisements and inconsistent
decisions among routers. OSPF addresses the following issues:
- Speed of convergence
- Support for Variable
Length Subnet Mask (VLSM)
- Network size
- Path
selection
- Grouping of members
In large
networks RIP convergence can take several minutes since the
routing table of each router is copied and shared with directly
connected routers. After initial OSPF convergence, maintaining
a converged state is faster because only the changes in the
network are flooded to other routers in an area. OSPF supports
VLSMs and therefore is referred to as a classless protocol. RIP
v1 does not support VLSMs, however, RIP v2 does support VLSMs.
RIP considers a network that is more than 15 routers away to be
unreachable because the number of hops is limited to 15. This
limits RIP to small topologies. OSPF has no size limits and is
suitable for intermediate to large networks. RIP selects a path
to a network by adding one to the hop count reported by a
neighbor. It compares the hop counts to a destination and
selects the path with the smallest distance or hops. This
algorithm is simple and does not require a powerful router or a
lot of memory. RIP does not take into account the available
bandwidth in best path determination. OSPF selects a path using
cost, a metric based on bandwidth. All OSPF routers must obtain
complete information about the networks of every router to
calculate the shortest path. This is a complex algorithm.
Therefore, OSPF requires more powerful routers and more memory
than RIP. RIP uses a flat topology. Routers in a RIP region
exchange information with all routers. OSPF uses the concept
of areas. A network can be subdivided into groups of routers.
In this way OSPF can limit traffic to these areas. Changes in
one area do not affect performance in other areas. This
hierarchical approach allows a network to scale efficiently.
Interactive Media Activity Checkbox: Link-State and
Distance Vector Comparison When the student has completed this
activity, the student will be able to identify the difference
between link-state and distance vector routing protocols.
Web Links OSPF versus RIP
http://www.cisco.com/warp/public/ 104/2.html#1.1
Content
2.2 Single Area OSPF Concepts 2.2.4 Shortest
path algorithm The shortest path algorithm is used by OSPF to
determine the best path to a destination. In this algorithm,
the best path is the lowest cost path. The algorithm was
discovered by Dijkstra, a Dutch computer scientist, and was
explained in 1959. The algorithm considers a network to be a
set of nodes connected by point-to-point links. Each link has a
cost. Each node has a name. Each node has a complete database
of all the links and so complete information about the physical
topology is known. All router link-state databases are
identical. The table in Figure shows the information that node
D has received. For example, D received information that it was
connected to node C with a link cost of 4 and to node E with a
link cost of 1. The shortest path algorithm then calculates a
loop-free topology using the node as the starting point and
examining in turn information it has about adjacent nodes. In
Figure , node B has calculated the best path to D. The best
path to D is by way of node E, which has a cost of 4. This
information is converted to a route entry in B which will
forward traffic to C. Packets to D from B will flow B to C to
E, then to D in this OSPF network. In the example, node B
determined that to get to node F the shortest path has a cost
of 5, via node C. All other possible topologies will either
have loops or a higher cost paths. Web Links Shortest
Path Algorithm http://www.cisco.com/warp/public/
104/2.html#2.0
Content 2.2 Single Area OSPF
Concepts 2.2.5 OSPF network types A neighbor
relationship is required for OSPF routers to share routing
information. A router will try to become adjacent, or neighbor,
to at least one other router on each IP network to which it is