Prohibiting The Advertisement Of Networks; Generating A Default External Route (Asbr) - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

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Take care to define the network that the ABR and the stub router have in
common as part of the stub area. Otherwise, each stub router becomes an
ABR. Because routers in the same area must use the same database, the
routers in the stub areas would then hold all area 0 information (or, in other
words, the topology of the entire WAN). You have, in effect, negated the
partitioning.

Prohibiting the Advertisement of Networks

You can prohibit an ABR from advertising networks in one area to routers in
another area. You can also prohibit the advertisement of only a certain range
of destinations within the area. Simply configure two route summaries for the
area: one for accessible networks with the advertise keyword and one for
prohibited networks with the not-advertise keyword.
For example, you could prohibit a router from advertising a summary link to
networks ranging from 192.168.1.0 to 192.168.63.0 with the following
command:
ProCurve(config-ospf)# area 1 range 192.168.0.0 255.255.192.0 not-advertise
As long as their ABR has a route to the network(s), hosts will still be able to
reach these networks using their default route. However, other routers will
not hold an actual route to the network(s).

Generating a Default External Route (ASBR)

An ASBR is a router on an OSPF network that also connects to an external
network. Typically, it runs both OSPF and an exterior gateway protocol.
However, because OSPF defines all non-OSPF routes as external, the ASBR
may simply be a WAN router with a default route that it redistributes into
OSPF.
The ASBR needs to somehow make the external routes that it learns available
to other OSPF routers. A common way of doing this is to advertise a default
route for external routes.
From the OSPF configuration mode context on the ASBR, enter this
command:
Syntax: default-information-originate [always] [metric <value>] [metric-type {1 | 2}]
IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring OSPF
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