| 9.5 | Redundancy, Symmetry, and Load Balancing | ||
| 9.5.3 | Default routing in BGP networks |
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You have 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, you could be receiving default information from multiple BGP
sources. In a BGP system, the Local Preference attribute can be
manipulated on the various default routes so that at least one default
route is identified as primary (that is, 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: RTC(config)#router bgp 3 If RTC is configured as shown above, it will send default information only to the specified neighbor. If you want to configure a BGP router to advertise a default to all of its peers, you can use the network command as shown below. Both neighbors -- 172.16.20.1 and 172.17.1.1 -- will receive a default route from RTC (if it has one to send). RTC(config)#router bgp 3 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 you more control over routing within an AS.
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