OSPF Routing Fundamentals
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OSPF stands for Open Shortest Path First.
Definition: OSPF is a routing protocol used to determine the best route for
delivering the packets within an IP networks. It was published by the IETF to
serve as an Interior Gateway Protocol replacing RIP. The OSPF specification is
published as Request For Comments (RFC) 1247.
Note that OSPF is a link-state routing protocol, whereas RIP and IGRP are
distance-vector routing protocols. Routers running the distance-vector algorithm
send all or a portion of their routing tables in routing-update messages to
OSPF sends link-state advertisements
(LSAs) to all other routers within the
same area. Information on attached interfaces, metrics used, and other variables
is included in OSPF LSAs. OSPF routers use the SPF (Shortest Path First)
algorithm to calculate the shortest path to each node. SPF algorithm is also
known as Dijkstra algorithm.
Advantages of OSPF
OSPF is an open standard, not related to any particular vendor.
OSPF is hierarchical routing protocol, using area 0 (Autonomous System) at
the top of the hierarchy.
OSPF uses Link State Algorithm, and an OSPF network diameter can be much
larger than that of RIP.
OSPF supports Variable Length Subnet Masks
(VLSM), resulting in efficient
use of networking resources.
OSPF uses multicasting within areas.
After initialization, OSPF only sends updates on routing table sections
which have changed, it does not send the entire routing table, which in turn
conserves network bandwidth.
Using areas, OSPF networks can be logically segmented to improve
administration, and decrease the size of routing tables.
Disadvantages of OSPF:
OSPF is very processor intensive due to implementation of SPF algorithm.
OSPF maintains multiple copies of routing information, increasing the amount
of memory needed.
OSPF is a more complex protocol to implement compared to RIP.
OSPF Networking Hierarchy:
As mentioned earlier, OSPF is a hierarchical routing protocol. It enables
better administration and smaller routing tables due to segmentation of entire
network into smaller areas. OSPF consists of a backbone (Area 0) network that
links all other smaller areas within the hierarchy. The following are the
important components of an OSPF network:
Area Border Routers
AS Boundary Routers
Totally Stubby Area
ABR: Area Border Router
ASBR: Autonomous System Boundary Router
Areas: An area consists of routers that have been administratively grouped
together. Usually, an area as a collection of contiguous IP subnetted networks.
Routers that are totally within an area are called internal routers. All
interfaces on internal routers are directly connected to networks within the
Within an area, all routers have identical topological databases.
Area Border Routers: Routers that belong to more than one area are called
area border routers (ABRs). ABRs maintain a separate topological database for
each area to which they are connected.
Backbone Area: An OSPF backbone area consists of all routers in area 0,
and all area border routers (ABRs). The backbone distributes routing
information between different areas.
AS Boundary Routers
(ASBRs): Routers that exchange routing information with
routers in other Autonomous Systems are called ASBRs. They advertise externally
learned routes throughout the AS.
Stub Areas: Stub areas are areas that do not propagate AS external
advertisements. By not propagating AS external advertisements, the size of
the topological databases is reduced on the internal routers of a stub area.
This in turn reduces the processing power and the memory requirements of the
(NSSA): An OSPF stub area has no external routes in it. A
NSSA allows external routes to be flooded within the area. These routes are then
leaked into other areas. This is useful when you have a non-OSPF router
connected to an ASBR of a NSSA. The routes are imported, and flooded throughout
the area. However, external routes from other areas still do not
enter the NSSA.
Totally Stubby Area: Only default summary route is allowed in Totally Stubby
Transit Areas: Transit areas are used to pass traffic from an adjacent area
to the backbone. The traffic does not originate in, nor is it destined for, the
Link State Advertisements
It is important to know different Link State Advertisements
(LSAs) offered by
Type 1: Router link advertisements generated by each router for each area it
belongs to. Type 1 LSAs are flooded to a single area only.
Type 2: Network link advertisements generated by designated routers
giving the set of routers attached to a particular network. Type 2 LSAs are
flooded to the area that contains the network.
Type 3/4: These are summary link advertisements generated by ABRs describing
inter-area routes. Type 3 describes routes to networks and is used for
summarization. Type 4 describes routes to the ASBR.
Type 5: Generated by the ASBR and provides links external to the Autonomous
System (AS). Type 5 LSAs are flooded to all areas except stub areas and totally
Type 6: Group membership link entry generated by multicast OSPF routers.
Type 7: NSSA external routes generated by
ASBR. Only flooded to the NSSA. The
ABR converts LSA type 7 into LSA type 5 before flooding them into the backbone
Type 5 AS-external LSA NOT allowed
Type 5 AS-external LSAs are NOT allowed, but Type 7 LSAs that convert
to Type 5 at the NSSA ABR can traverse
Type 3, 4 or 5 LSAs are NOT allowed except the default summary route