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Autonomous System Boundary Routers 459
Router placement
and participation
When you populate an area with OSPF routers, consider the following
guidelines:
■
Because OSPF uses a CPU-intensive algorithm, keep the maximum
number of routers participating in OSPF exchanges in any given area
to around 50. This number decreases the likelihood of performance
problems that may be associated with router recalculation. If the link is
of high quality and the number of routes is minimal, you can increase
the number of area routers.
■
Keep the maximum number of neighbors for any one router to around
60. Each time that a topological change occurs, a router exchanges
information only with those neighbors with which it has formed an
adjacency. On a multiaccess network, this neighbor count is only of
concern to the designated and backup-designated router of an area
because, on a multiaccess network, area routers do not exchange
link-state information with each other. Instead, they exchange
link-state information with only the designated and backup
designated routers.
Autonomous
System Boundary
Routers
Autonomous system boundary routers (ASBRs) are the links between the
OSPF autonomous system and the outside network. They exchange their
autonomous system topology data with boundary routers in other
autonomous systems.
ASBRs can import external link advertisements that contain information
about external networks from other protocols like RIP and redistribute
them as LSAs to the OSPF network. In this way, ASBRs flood information
about external networks to routers within the OSPF network.
Every router inside an autonomous system knows how to reach the
boundary routers for its autonomous system.
In Figure 54, two ASBRs control traffic between two autonomous
systems.
Configuring an ASBR A router becomes an ASBR as a by-product of other settings. A router
becomes an ASBR if any operational in-band IP interface on the router:
■
Has both OSPF and RIP disabled on that interface. Or,
■
Has RIP configured as learn, advertise, or enabled on that interface.