NOTE:
The routing switch is configured, by default, to be compliant with the RFC 1583 OSPF V2
specification. (Use
show ip ospf general
to view the current RFC 1583 configuration setting.)
All routes in an AS should be configured with the same compliance setting for preference rules
affecting external routes. Thus, if any routers in an OSPF domain support only RFC 1583, all routers
must be configured with 1583 compatibility. In the default OSPF configuration, RFC 1583 support is
enabled for the routing switches.
If all routers in the domain support RFC 2178 or RFC 2328, you should disable RFC 1583 compatibility on all of
the routers, because conformance to these later RFCs provides more robust protection against routing loops on
external routes.
Assigning the routing switch to OSPF areas
After you globally enable OSPF on the routing switch (see
Changing the RFC 1583 OSPF compliance setting
on page 258), use this command to assign one or more OSPF areas within your AS. A routing switch can belong
to one area or to multiple areas. (Participation in a given, assigned area requires configuring one or more VLANs
or subnets and assigning each to the desired area.
• If you want the VLANs and any subnets configured on the routing switch to all reside in the same area, you
need to configure only that one area. (In this case, the routing switch would operate as an internal router for
the area.)
• If you want to put different VLANs or subnets on the routing switch into different areas, you need to re-execute
this command for each area. (In this case, the routing switch will operate as an ABR for each of the configured
areas.)
NOTE:
Each ABR must either be directly connected to the backbone area (0) or be configured with a virtual
link to the backbone area through another ABR that is directly connected to the backbone area. See
Configuring an ABR to use a virtual link to the backbone
on page 261.
Configuring for external route redistribution in an OSPF
domain
Configuring route redistribution for OSPF establishes the routing switch as an ASBR (residing in a backbone,
normal, or NSSA) for importing and translating different protocol routes from other IGP domains into an OSPF
domain. The switches support redistribution for static routes, RIP routes, and directly connected routes from RIP
domains into OSPF domains. When you configure redistribution for OSPF, you can specify that static, connected,
or RIP routes external to the OSPF domain are imported as OSPF routes. (Likewise, RIP redistribution supports
the import of static, connected, and OSPF routes into RIP routes.) The steps for configuring external route
redistribution to support ASBR operation include the following:
Procedure
1.
Configure redistribution filters to exclude external routes that you do not want redistributed in your OSPF
domain.
2.
Enable route redistribution.
3.
Modify the default metric for redistribution (optional.)
4.
Modify the redistribution metric type (optional.)
5.
Change the administrative distance setting (optional.)
Chapter 11 Open Shortest Path First Protocol (OSPF)
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