Multi Area OSPF


Multi-area OSPF allows OSPF to scale to fit considerably larger networks than Single Area OSPF can effectively service. In a multi-area OSPF configuration, routers can be grouped into four distinct catagories. See worksheet for practice.

Router Type Description
Internal Router All router interfaces are in a single area
Backbone Router At least one interface is in area 0
Area Border Router (ABR) This router is attached to multiple areas
Autonomous System Boundary Router (ASBR) At least one interface is connected to another autonomous system


Additionally, there are a total of five different area types for Cisco routers

Area Type Description
Standard A standard area accepts link updates and route summaries. This is the same as the area covered in Single Area OSPF
Backbone (transit) This is area 0. All other areas must connect to this, as it conveys all information between areas.
Stub Area A stub area doesn't accept routes to other autonomous systems. Instead it uses a default route (0.0.0.0/0)
Totally Stubby Like the stub area, totally stubby areas don't accept routes to other autonomous systems. Additionally, totally stubbies don't accept routes to other areas. A default route is used instead (0.0.0.0/0). Totally Stubby is a Cisco proprietary area.
Not So Stubby (NSSA) A totally stubby area is a special kind of stub that exists only with an ASBR. This allows importing of external routes via type 7 LSAs, which are then translated to type 5 at the ABR.


Link State Advertisements (LSAs) are used to update link-state tables.
An LSA is encapsulated in a type 4 OSPF packet (LSU) as covered in Single Area OSPF.
There are a total of 7 different LSAs as follows.


LSA Type Abbreviation Description
1. Router Link Entry (O-OSPF) This is generated by all routers for routers within their area. This is used within a single area only.
2. Network Link Entry (O-OSPF) This is generated by a DR for their specific, multiaccess area. This is used within a single area only.
3. Summary Link Entry (IA-OSPF) This is generated by an ABR and sent to the backbone and other ABRs. This describes the links internal to an ABR's specific area. This is used between areas only.
4. Summary Link Entry (IA-OSPF) This is generated by an ABR and sent to the backbone and other ABRs. This describes how to reach ASBRs located within the ABR's area. These are not sent to totally stubby areas. Inter-area only.
5. Autonomous System External Link Entry (E1-OSPF external type 1)
(E2-OSPF external type 2)
This is sent from an ASBR to an OSPF autonomous system describing external routes. These updates are not allowed in stub, totally stubby, or NSSA areas. E1 is for multiple routes to an external entity (multihomed AS) and the cost = external + internal cost. E2 is for only one external route (singlehomed AS) and the cost = external cost. E2 is the default.
6. Multicast OSPF (MOSF) This is not implemented by Cisco. It allows multicast distribution trees to forward multicast packets.
7. Autonomous System External Link Entry (N1-OSPF NSSA type 1)
(N2-OSPF NSSA type 2)
These LSAs are only sent by an ASBR that is part of a Not So Stubby area (NSSA). It is then translated to type 5 LSAs at the ABR before continueing through the OSPF autonomous system (AS).


When calculating its routing table, an multiarea OSPF router goes through several stages to find best routes.
These are summarized below.


Step Area Description LSAs used
1. Local Area Calculate all paths within own area. Type 1 and Type 2
2. Inter-Area Calculate all paths to other areas. Type 3 and Type 4 (not totally stubby)
3. External Calculate all paths to external ASes. Type 5 or 7 (not stub)




Configuring ABRs and ASBRs

ABR -
An ABR is automatically configured when you place router interfaces into multiple areas e.g.

router OSPF 1
network 10.0.0.0 0.255.255.255 area 1
network 10.0.0.0 0.255.255.255 area 0


ASBR -
An ASBR is automatically configured when you set OSPF to import external routes, e.g.

router rip
network 10.0.0.0
router OSPF 1
network 192.168.1.0 0.0.0.255 area 0
redistribute rip



Route Summarization

A route summarization is used to summarize multiple routes with a single route entry. In addition to reducing the size of a routing table, this also prevents route flapping from affecting other areas; a summary route will still be valid even if one of its component routes goes down. This will reduce SPF recalculation in case routes go down, reducing processor overhead.

Interarea-
Interarea route summarization is done by an ABR and applies to routes within each area. So set up interarea route summarization, use the following command:

area [area-id] range [address mask]
e.g.
router ospf 1
area 1 range 192.168.16.0 255.255.252.0


External-
External route summarization is done by an ASBR and applies to external routes injected into the AS via redistribution. To set up external route summarization, use the following command:

summary-address [address mask]
e.g.
router ospf 1
summary-address 206.9.0.0 255.255.0.0



Stub and Totally Stubby Area

Stub and Totally Stubby Areas are used to reduce the number of LSAs and route calculations an area needs to make. This is handy if the hardware is being heavily taxed by external routes, or if LSAs are significantly reducing network bandwidth. The following criteria should be met to configure a stub or totally stubby area.

Only one exit point from the area
Not needed as a transit area for virtual links
No ASBR internal to area
Not Area 0


To configure a stub or totally stubby area, use the following commands:


Command Description
router OSPF 1 Enter OSPF configuration
area [area-id] stub Set an area as stub. Must be done on all routers within a stub (or totally stubby) area.
area [area-id] stub no-summary Set an area as totally stubby. This command is only done on the ABR.
area [area-id] default-cost [cost] Optional. This is done on an ABR only and sets the default cost for the static route being given to stub routers within the stub area.



Virtual Links

As covered previously, all areas must be attached to Area 0 via an ABR. In some instances, it is impossible to directly attach an ABR to Area 0. A virtual link can be used to attach the ABR to area 0 virtually. Note that this can only be done if an area is one area separated from area 0. If there are more areas separating the two, this will not work and a network redesign needs to be done. Virtual links can also be used to patch two area 0s into one, contigous backbone. This can be handy if merging two OSPF networks into one.

The command is as follows and should be executed on the ABR:

router OSPF1
area [area-id] virtual-link [router-id]

A router id can be determined by running a
'show ip ospf' command. Following is an example virtual link configuration.





Not So Stubby Areas (NSSA)

An NSSA is a special type of stub network. It is only used in an area with an ASBR, and is typically implemented to reduce routing traffic and SPF calculations within that area while maintaining communications with external ASes. External routes injected into the NSSA by the ASBR use type 7 packets. When the type 7 packets hits the ABR, it is then translated to a type 5 packet for dissemination through the rest of the OSPF AS.

To configure an NSSA, use the following commands:

area [area-id] nssa [no-summary]

Usually you would want to use the no-summary keyword, which prevents type 3 and type 4 LSAs. This further reduces OSPF traffic and SPF recalculations. You may also wish to use route summarization on the ASBR to further reduce SPF calculations and routing table entries.

Summary-address [prefix mask] [not advertise] [tag tag]

The [not advertise] option is used to suppress routes that match the prefix/mask. This only works for OSPF routes. [tag] is used for policy routing.

Multi-Area OSPF Show Commands

Command Description
show ip ospf border-routers Display internal OSPF routing table entries to an ABR
show ip ospf virtual-links Display virtual link parameters
show ip ospf [process-id] display are information and router OSPF information
show ip ospf database Show the topological database
show ip ospf [process-id area-id] database [router] Show router link-state information
show ip ospf [process-id area-id] database [network] Show network link-state information
show ip ospf [process-id area-id] database [summary] Summary information about ABR link-states
show ip ospf [process-id area-id] database [asbr-summary] Information on ASBR link states
show ip ospf [process-id area-id] database [external] Information on AS external link states
show ip ospf [process-id area-id] database [database-summary] display database summary and totals


LSA and area type summary

LSA Type Description
1. Router link entry Internal to 1 area
2. Network Link entry Internal to 1 area, sent by DR in multiaccess networks
.
3. Summary Link Entry ABR to ABR in an AS and to backbone. Describes links in AS
4. Summary Link Entry ABR to ABR in an AS and to backbone. Describes links to ASBR(s).
.
5. AS External Link From ASBR to all except in NSSA and stubs. E1/E2.
7. AS External Link From ASBR to ABR, only in an NSSA. N1/N2.
.
6. Multicast Not Used

Area Type LSAs allowed
Standard multiaccess 1,2,3,4,5
Standard not-multiaccess 1,3,4,5
Backbone 1,2,3,4,5
Stub 1,2,3,4
Totally Stubby 1,2
Not So Stubby 1,2,3,4,7
Not So Stubby, no-summary 1,2,7





References
http://clankiller.com/technical/guides/cisco/ciscomaospf/index.php