Multi Area OSPF (page 2)
26 Oct 2003 @ 08:14AM
by Satis

Updated: 20 Jan 2010 @ 09:10AM
Configuring ABRs and ASBRs

An ABR is automatically configured when you place router interfaces into multiple areas. Eg;

router OSPF 1
network area 1
network area 0

An ASBR is automatically configured when you set OSPF to import external routes. Eg;

router rip
router OSPF 1
network area 0
redistribute rip
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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 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]

Router OSPF 1
area 1 range

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]

router ospf 1
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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:

router OSPF 1Enter OSPF configuration
area [area-id] stubSet an area as stub. Must be done on all routers within a stub (or totally stubby) area.
area [area-id] stub no-summarySet 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.
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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.

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