MultiArea OSPF Flashcards
Why Multiarea OSPF?
Issues in a large single area OSPF:
• Large routing table
• Large link-state database (LSDB)
• Frequent SPF algorithm calculations
To make OSPF more efficient and scalable, OSPF supports hierarchical routing using areas.
Multiarea OSPF
Multiarea OSPF:
• Large OSPF area is divided into smaller areas.
• Reduces processing and memory overhead.
• Requires a hierarchical network design.
• The main area is the backbone area (area 0) and all other areas connect to it.
Advantages of Multiarea OSPF:
• Smaller routing tables - Fewer routing table entries as network addresses can be summarized between areas.
• Reduced link-state update overhead.
• Reduced frequency of SPF calculations.
Two Layers Area Hierarchy
Backbone (Transit) area
- An OSPF area whose primary function is the fast and efficient movement of IP packets:
• Interconnects with other OSPF area types.
• Also called OSPF area 0.
Regular (nonbackbone) area
- Connects users and resources:
• Usually set up along functional or geographical groupings
• All traffic from other areas must cross a transit area.
Types of OSPF Routers
• Internal router –A router that has all of its interfaces in the same area.
• Backbone router - A router in the backbone area. The backbone area is set
to area 0
• Area Border Router (ABR) – A router that has interfaces attached to multiple
areas.
• Autonomous System Boundary Router (ASBR) – A router that has at least
one interface attached to an external internetwork.
A router can be classified as more than one router type.
Types of Routes
O- intra area
O IA -summery LSA
O E1 -external area
O E2- external area
What is difference between E1 and E2 route?
E1 has increasing cost while E2 is equaly on all routers
LSA type1 : Router LSA
• One router LSA (type 1) for every router in an area
• Includes list of directly attached links
• Each link identified by IP prefix assigned to link and link type
• Identified by the router ID of the originating router
• Floods within its area only; does not cross ABR
LSA Type 2: Network LSA
• One network (type 2) LSA for each transit broadcast or NBMA network in an area
• Includes list of attached routers on the transit link
• Includes subnet mask of link
• Advertised by the DR of the broadcast network
• Floods within its area only; does not cross ABR
LSA Type 3: Summary LSA
• Type 3 LSAs are used to flood network information to areas outside the originating
area (interarea)
• Describes network number and mask of link.
• Advertised by the ABR of originating area.
• Regenerated by subsequent ABRs to flood throughout the autonomous system.
• By default, routes are not summarized, and type 3 LSA is advertised for every subnet.
LSA Type 4: Summary LSA
• Summary (type 4) LSAs are used to advertise an ASBR to all other areas in the
autonomous system.
• They are generated by the ABR of the originating area.
• They are regenerated by all subsequent ABRs to flood throughout the
autonomous system.
• Type 4 LSAs contain the router ID of the ASBR.
LSA Type 5: External LSA
• External (type 5) LSAs are used to advertise networks from other autonomous systems.
• Type 5 LSAs are advertised and owned by the originating ASBR.
• Type 5 LSAs flood throughout the entire autonomous system.
• The advertising router ID (ASBR) is unchanged throughout the autonomous system.
• Type 4 LSA is needed to find the ASBR.
• By default, routes are not summarized.
Types of areas
An area can be stub or totally stub if:
• There is a single ABR, or if there is more than one ABR, suboptimal routing paths to other areas or external autonomous systems are acceptable.
• All routers in the area are configured as stub routers.
• There is no ASBR in the area.
• The area is not area 0.
• No virtual links go through the area.
Stub Areas
• External LSAs are stopped.
• Default route is advertised into stub area by the ABR.
• All routers in area 50 must be configured as stub.
Totally Stubby Areas
• External LSAs are stopped.
• Summary LSAs are stopped.
• Routing table is reduced to a minimum.
• All routers must be configured as stub.
• ABR must be configured as totally stubby.
• This is a Cisco proprietary feature.
Defining Virtual Links
• Virtual links are used to connect a discontiguous area to area 0.
• A logical connection is built between router A and router B.
• Virtual links are recommended for backup or temporary connections.
Benefits of Route Summarization
• Minimizes number of routing table entries
• Localizes impact of a topology change
• Reduces LSA type 3 and 5 flooding and saves CPU resources
Default Routes in OSPF
• A default route is injected into OSPF as an external LSA type 5.
• Default route distribution is not on by default; use the default-
information originate command under the OSPF routing process.
• Normally, this command advertises a 0.0.0.0 default into the OSPF
network only if the default route already exists in the routing table.
• The always keyword allows the 0.0.0.0 default to be advertised even
when the default route does not exist in the routing table.
with metric we can increase cost and change priority of default routes
