All of my customers run EIGRP. What
that means is that the networks that I work on only use Cisco routers, so I
can run Cisco's proprietary protocol. This means that I don't have a
clue when it comes to Open Shortest Path First (OSPF). Not a good thing on
the path to the CCIE. This week's newletter outlines some of the
fundamentals of OSPF.
OSPF is a link state routing protocol that updates
the routing table only when network changes occur, rather than at a
predefined interval. OSPF supports variable length subnet masks (VLSM) and
route summarization.
OSPF routers and links are grouped logically into
areas that are identified by assigned numbers. All OSPF networks have at
least one area with the default being area 0. If more than one area exists,
area 0 is defined as the backbone area and is used to connect all other
areas. Each area has its own link state databases.
Stub areas are
OSPF areas that have default routes, internal and inter-area routes, but do
not flood external routes. A stub area has no routers or areas beyond
it.
Totally Stubby Areas are stub areas that do not carry inter-area
routes (LSA types 3/4) as well.
A Not So Stubby Area is a hybrid area
used to connect two disparate organizations. External routes are advertised
using LSA type 7 that are less demanding on router resources.
Different types of OSPF routers
Internal Router:
Responsible for maintaining a current and accurate database of subnets
within the area. Forwards data to other networks using the shortest
path.
Backbone Router: Has an interface connected to the backbone
(Area 0).
Area Border Router (ABR): Has interfaces in multiple areas
with at lest one interface in area 0. Connects other areas to the
backbone and maintains routing information for each connected
area.
Autonomous System Boundary Router (ASBR): Router located
between OSPF autonomous system and a non-OSPF network. Used to
redistribute routing information between networks. Must reside in a non-stub
area.
OSPF LSA Types
Type 1: Router link advertisements
generated by each router for each area it belongs to. Flooded to a single
area only.
Type 2: Network link advertisements generated by designated
routers describing the set of routers attached to a particular network.
Flooded to the area that contains the network.
Type 3/4: 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 describes links
external to the Autonomous System (AS). Flooded to all areas except stub
areas.
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 (area 0).
Realizing where each component
of an OSPF network fits is the first step to understanding how to
effectively use OSPF. Cisco provides lots of OSPF design information and
advice. Check out the following links for more information:
Email this Article
Print this Article
