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9.10 | ![]() |
BGP Redistribution | |
9.10.2 | ![]() |
Injection of unwanted or faulty information |
Injecting routes into BGP by way of the network command may not always
be practical or even possible. Injecting routes by way of
redistribution can result in polluting other autonomous systems with
unwelcome, incorrect, or otherwise undesirable information.
Redistributing the entire IGP table into BGP could result in private
addresses, or illegal addresses being advertised outside the AS. In some cases, routes with
inappropriate prefix lengths could make it upstream to the provider
where they are not needed. For example, host routes are generally
greeted with disdain by annoyed systems administrators.
Mutual redistribution between IGP and BGP can also result in the
propagation of flawed routing information. In this case, a BGP route
that was injected from the outside could be sent back into BGP by way
of the IGP. This happens as if the route originated within the AS. Figure
In Figure
To remedy these situations, special filtering should be put on the border routers to specify what particular networks should be injected from the IGP into BGP. For protocols that differentiate between internal and external routes, such as OSPF, configure the IGP to ensure that it will redistribute only internal routes into BGP. In the Cisco implementation, external OSPF routes are automatically blocked from being redistributed into BGP. There is the option of overriding this behavior. Certain protocols may not distinguish between internal and external routes, such as RIP or IGRP. For these types of protocols, special route tagging should be performed to differentiate between external routes and internal routes.
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