CO5023
Multi Area OSPF
Limitations of single area OSPF
• When networks become large,
single area OSPF develops
problems
• Routing table becomes large
• Link state database becomes large also
as it stores the topology of the entire
network
• Frequent SPF calculations
•
The larger the network the more frequent
the changes in topology.
• The solution to this is multi area OSPF:
where we divide the network into
multiple areas. Each router computes the
topology for the areas it is attached to
• The scheme is hierarchical, we have a
backbone area (area 0) which all other
regular areas must connect to.
Multi-area rules and terminology
Rules…
1. All inter area traffic must cross the backbone area
2. An area should have no more than 50 routers
3. A router should not be in more than 3 areas
4. A single router should not have more than 60
neighbours
Terminology
• An Internal router is one which belongs to a
single non-backbone areas
• A Backbone router has all interfaces in the
backbone area
• An Area Border Router (ABR) has interfaces in
more than one area
• An Autonomous System Boundary Router
(ASBR) has a connection to an external
internetwork
LSA Types
1. Lists directly connected network and
prefix types
2. For multi-access networks, generated by
Designated Router.
3. Used by ABRs to advertise networks in
other areas. Summarised where possible
4. Used by the ABR to identify the ASBR
and provide a route to it
5. Used by ASBR to describe routes to
external networks
Multi-area routing table
O - Router (type 1) and network (type 2) LSAs describe
the details within an area (intra-area).
O IA - When an ABR receives summary LSAs, it adds
them to its LSDB and regenerates them into the local
area. Summary LSAs appear in the routing table as IA
(inter-area routes).
O E1 or O E2 - External LSAs appear in the routing table
marked as external type 1 (E1) or external type 2 (E2)
routes.
The SPF calculation starts with intra-area routes, then
inter-area, then external.
Multi area configuration…
OSPF Route Summarisation
Configure Route Summarisation