| 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 |
| 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. |
| 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). |
| 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
| 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 |
| 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.| router ospf 1 | |
| area 1 range 192.168.16.0 255.255.252.0 |
| 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 |
| 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.| router OSPF1 |
| area [area-id] virtual-link [router-id] |
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.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 |