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9.9 | ![]() |
Redundancy, Symmetry, and Load Balancing | |
9.9.3 | ![]() |
Default routing in BGP networks |
It has been seen that default routes minimize the size of a routing
table. They can also provide networks with redundancy in the event of
failures and connectivity interruptions. BGP, like almost all IGPs,
can distribute a default route. To provide redundancy, default
information could be received from multiple BGP sources. In a BGP
system, the Local Preference attribute can be manipulated on the
various default routes. This is so that one default route is identified as a
primary, the highest Local Preference, and others are kept as backups.
If the primary fails, a backup route with the next highest preference
can take its place.
In Figure
In Figure
It is important to control default information in BGP because improper configuration can cause serious Internet routing problems. For example, a misconfigured BGP speaker could end up flooding a default route to all of its neighbors and quickly find itself consumed with default routed traffic from surrounding autonomous systems. To protect against misadvertisements, the Cisco IOS provides a way to target default information at a specific neighbor by using the default-originate option with the neighbor command:
If RTC is configured as shown in this configuration, it will send default information only to the specified neighbor. If a BGP router is to be configured to advertise a default to all of its peers, use the network command shown as follows:Note: Both neighbors, 172.16.20.1 and 172.17.1.1, will receive a default route from RTC, if it has one to send.
Many network administrators choose to filter dynamically learned default routes to avoid situations in which traffic ends up where it is not supposed to be. Without dynamically learned default routes, a router must be statically configured with default information. Statically configured default routes typically provide more control over routing within an AS.
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