OSPF in Multiple Areas
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Scalability Problems in Large
OSPF Areas
Scalability problems in large OSPF areas
include
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Large routing tables
Large topology databases
Frequent recalculation of Shortest Path First
(SPF) algorithm
Slow synchronization
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Advantages of Using Multiple
Areas
Advantages of using multiple OSPF areas
within an Autonomous System include
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Smaller routing tables
Less frequent SPF recalculations
Less routing update overhead
Faster synchronization
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Connecting OSPF Areas
To connect two OSPF areas, use a router
with at least one interface in both areas
OSPF uses different types of routers and
areas
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Types of Routers
Types of OSPF routers include:
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Internal routers
Backbone routers
Areas Border Routers (ABRs)
Autonomous System Border Routers (ASBRs)
A router may simultaneously be several types of
routers
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Keep in mind the cost of performing each function
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The Backbone Area
Backbone area is most important area in
any OSPF autonomous system
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It behaves like a standard area
All other areas connect to it to exchange
routing information
All packets sent between areas go through it
Backbone area must remain stable and
reachable
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Other Types of Areas
Classified by interaction with each other
Standard area – accepts link updates and
route summaries internally and externally
Stub area – does not accept routing
information from ASBRs; uses a default
route to reach external devices
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Other Types of Areas
Totally stubby area – does not accept external
routes and refuses summary routes from other
areas; uses a default route; Cisco's proprietary
extension to OSPF protocol
Not-so-stubby-area (NSSA) – accepts limited
information about external routes, but refuses
routing information from ASBRs; useful for
connecting stub area to another Autonomous
System
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Not-so-stubby Area
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Routing Between Areas
Routing between multiple areas adds
complexity to OSPF
Must first understand different types of
LSAs used to describe topology of a
network
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Types of LSAs
• OSPF uses several types of LSAs to
describe topology of a network
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Router link entry LSA (LSA type 1)
Network link entry LSA (LSA type 2)
Summary link entry LSA (LSA types 3 and 4)
Autonomous System external link entry LSA
(type 5)
NSSA Autonomous System external link entry
LSA (type 7)
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Types of LSAs
continued
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Types of LSAs
Type 6 LSA is not included because CISCO routers do not support it
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Routes Between Multiple Areas
• If packet’s destination is inside same area
as source, it passes through internal
routers
• If packet’s destination is outside area of
source, it goes to an ABR in its own area
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ABR sends it through backbone to an ABR in
destination area
• OSPF Autonomous System uses a variety
of types of routes
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Calculating Routing Tables and
Flooding in Multiple Areas
ABRs must learn about routes and network
changes in other areas
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Within an area, each router receives LSUs describing
changes and then sends summary LSAs to any
attached areas
When an ABR or ASBR receives a summary LSA, it
adds information to link-state database and floods
local areas. Totally stubby areas do not receive
LSAs.
After all routers receive summary LSAs, they
recalculate their routing tables
See Figure 4-3
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Virtual Links
Each area in an OSPF Autonomous
System must be connected to backbone
If no physical link exists, OSPF uses
virtual link or a logical link
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Connects two ABRs via a common area,
sometimes called the transit area, and one
ABR attached to backbone
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Adding an Area with a Virtual
Link
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Purposes of Virtual Links
• Virtual links have three purposes
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Connecting areas to backbone when they
don’t have an ABR with an interface in
backbone area
Connecting backbone itself in situations
where it has been partitioned
Adding a redundant connection to backbone
to prevent failure of single link or backbone
area itself
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Designing Areas
Factors to consider when designing
OSPF areas include:
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Size of an area
Number of neighbors each router has
Problem isolation
Restriction of routing information
Stable backbone design
Addressing area
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Size of an Area
• Size of area affects number of routes in
routing table and frequency of SPF
calculations
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Topology, memory, and processors in router
determine appropriate sizes of areas
With high-speed links and stable network, may
have areas with up to 100 routers
If your network has slow unstable WAN links,
should use smaller areas
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Cisco recommends ABR have interfaces in no
more than three areas
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Number of Neighbors Per
Router
Number of neighbors affects performance
of router
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A router exchanges hello packets with its
neighbors and link-state databases with those
neighbors with which it forms adjacencies
Cisco recommends routers have no more
than 60 neighbors
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Isolating Potential Network
Problems
OSPF allows you to isolate potential
network problems to prevent them from
affecting other areas of the network
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Route flapping on WAN links causes
instability on OSPF networks
Use areas to isolate WAN links
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Isolating WAN Segments
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Restricting Routing Information
From an Area
Restricting routing information has several
advantages
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Reduces routing table size in an area
Reduces amount of flooding traffic after
topology change
Adds stability by isolating routers inside area
Configure an area as a stub area, a totally
stubby area, or a not-so-stubby area to
restrict routing information
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Selecting a Stub Area
Stub areas likely have only one exit point
Stub areas work well for branch offices or
hub-and-spoke topologies
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Spoke needs to get to hub and does not need to
know about rest of network
Stub area cannot be backbone area or transit
area for a virtual link
No need to have ASBRs within a stub network
since stubs do not accept external routes
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Totally Stubby Areas
Totally stubby areas reject external routes
and intra-area routes
Rely entirely on default routes to reach
destination outside totally stubby area
If area consists entirely of Cisco routers
and has a single exit point, using a totally
stubby area allows smaller routing tables
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Totally stubby areas are proprietary to Cisco
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Not-so-stubby Areas
Good for service provider
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Allows service provider’s network to connect
to customer network while preventing
customer network from incorporating into
OSPF Autonomous System
Good for redistributing other routing
protocols into OSPF Autonomous system
See Figure 4-7
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Example of NSSA
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Designating the Backbone Area
Stable backbone is essential
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Areas exchange routing information through
the backbone area
Design of the backbone area affects
stability of entire OSPF Autonomous
System
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Designing the Backbone Area
• Keep backbone area small to stabilize it
• Keep backbone area contiguous
• Provide redundant links between backbone
routers whenever possible
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See Figure 4-8
• Redundant links add complexity and cost,
but should be balanced against the cost of
a partitioned network
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See Figure 4-9
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Connecting Areas to the
Backbone with Virtual Links
• Use virtual links to prevent backbone area
from being partitioned or for redundancy
• Three reasons to avoid virtual links
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Virtual links prevent transit area from being
used as a stub area
Stability of virtual link depends on stability of
transit area
Using virtual links adds complexity to network
design and makes troubleshooting more
difficult
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Using WAN Links in the
Backbone Area
• In star topology, keep WAN links out of
backbone to prevent flapping
• In extended star topology, use WAN links
to connect hubs
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Avoid single links between backbone routers
Add redundant links to prevent single point of
failure
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Avoid Other Devices in
Backbone Area
• Avoid placing anything but routers in
backbone area
• Servers and workstations use bandwidth
needed for routing traffic and can cause
network to become unstable
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Malfunctioning network interface card or
IP address can duplicate router’s interface
IP address, causing serious problems
Other devices in backbone make
troubleshooting more difficult
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Addressing with Multiple Areas
Address selection determines how well
OSPF Autonomous System scales with
growth
Without route summarization, nonbackbone change requires routers in
backbone to recalculate routing tables
Minimize number of routes by carefully
using route summarization
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Route Summarization in OSPF
OSPF requires each router in an area to have
same knowledge of network
Two consequences for route summarization
 Routes cannot be summarized within an
area; would create different views of topology
 Summarization of routes from an area must
occur at ABRs since ABRs advertise routes
to other areas
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Summarizing routes
Summarize external routes with an ASBR or an
ABR
Summarize only contiguous blocks of IP
addresses
Summarize routes on ABRs connecting nonbackbone areas to backbone
Route summarization is useful for large
networks,
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Be sure all summarized routes are in same area
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Summarizing Routes
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Limits of Route Summarization
Route summarization with OSPF has limits
Can lead to poor path selection
May generate unnecessary traffic through
backbone
Does not allow you to summarize groups
of areas
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Addressing Areas
Consider numbering OSPF areas in
contiguous blocks
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Allows use of route summarization without
having to readdress network
Basing addressing on topology of network
or geographic location works well
Basing addressing on other schemes, such
as department structure of organization, can
lead
to problems
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Addressing Areas
• Assigning address blocks at an octet
boundary makes them easy to remember
and configure
• Keep thorough and accurate
documentation
• Avoid overlapping subnets or assigning
subnets in discontiguous blocks
• Assign IP addresses in contiguous blocks
based on router to which each subnet is
attached to facilitate route summarization
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Configuring OSPF in Multiple
Areas
Similar to configuring OSPF in a single
area, but adds layer of complexity
Activating multiple areas on an ABR
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Use router ospf statement to activate OSPF
Use network statement to put each interface
on the ABR into appropriate area
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Activating Multiple Areas on
an ABR
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Configuring Stub Areas
Each router in stub area must be
specifically configured as stub router
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Sets Stub Flag in hello packets
If Stub Flag does not match on two
neighboring routers, they cannot become
neighbors
Use area area-number command with stub
keyword
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Router(config-router) area 6 stub
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Configuring Totally Stubby
Areas
ABR is logical place to configure a totally stubby
area
Use area area-number command with stub and
no-summary keywords
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No-summary keyword must be used on each ABR in
a totally stubby area
Configure each router in totally stubby area with
area area-number stub command
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Router(config-router) area 6 stub no-summary
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Configuring Not-so-stubby
Areas
Each router in NSSA must be specifically
configured to be part of NSSA
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Use area area-number command with
keyword nssa
Router(config-router) area 6 nssa
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Default Routes
Configuring default routes varies from one area
to another
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In normal areas, use default-information originate
command
Can use router with a default route to propagate
default route to rest of OSPF Autonomous System
Add always keyword to configure a default route if
router does not have one
Can configure type of external link, cost of default route, and
identify a route map as default route
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Default-information Originate
Command
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Default Routes
In stub and totally stubby areas, ABRs
automatically generate default route using a
summary LSA
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No need to use default-information originate
command
In NSSA, an ABR must be forced to generate a
default route when you configure it with area
command
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Use default-information originate keyword
Use no-redistribution keyword to control route
redistribution
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Route Summarization
Configuring route summarization in OSPF
depends on whether you are summarizing
intra-area routers or external routers
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To summarize intra-area routers, use area
range command
To summarize an external route, use
summary-address command
Non-advertise keyword prevents advertising a
route
Tag keyword attaches a numeric tag value
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Configuring Virtual Links
Use area virtual-link command to
configure virtual link
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Routers at each end of virtual link must know
transit area and router ID at other end
See Figure 4-14
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Configuring a Virtual Link
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Monitoring and Troubleshooting
OSPF in Multiple Areas
• Use show ip ospf command to see what
area a particular interface is in and whether
the router is properly configured as an ABR
• Use show ip ospf interface to identify
whether an interface is in proper area
• Use show ip ospf database command
to see topology database of router
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Show IP OSPF Database
Command and Keywords
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Monitoring and Troubleshooting
OSPF in Multiple Areas
• Use show ip ospf border-routers
command to see internal OSPF routing
table entries to ABRs and ASBRs
• Use show ip route command to
troubleshoot route summarization
problems
• Use show ip ospf virtual-links
command to see status of virtual links, its
cost, and transit area
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