Multicast and Routing Guide K/KA/KB.15.15

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” (page 202).
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:
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.)
NOTE: Do not enable redistribution until you have used restrict to configure the redistribution
filters. Otherwise, your network might become overloaded with routes that you did not intend to
redistribute.
Configuring ranges on an ABR to reduce advertising to the backbone
Optional: Configuring ranges does the following to reduce inter-area advertising:
Summarizing routes Enable a routing switch operating as an ABR to use a specific IP
address and mask to summarize a range of IP addresses into a single
route advertisement for injection into the backbone. This results in
only one address being advertised to the network instead of all the
addresses within that range. This reduces LSA traffic and the resources
needed to maintain routing tables.
Blocking routes Prevent an ABR from advertising specific networks or subnets to the
backbone area.
Each OSPF area supports up to 8 range configurations.
Influencing route choices by changing the administrative distance default
(optional)
The administrative distance value can be left in its default configuration setting unless a change is
needed to improve OSPF performance for a specific network configuration.
The switch can learn about networks from various protocols, including RIP and OSPF. Consequently,
the routes to a network may differ depending on the protocol from which the routes were learned.
Configuring for external route redistribution in an OSPF domain 201