Content

| 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 illustrates the danger of mutual redistribution between protocols.

In Figure , AS100 is the source of NetA and is sending this information by way of BGP to AS200. The border router RTC injects that information into the IGP, and RTB learns about it. RTB is configured to redistribute the IGP information into BGP. NetA will end up being advertised by way of BGP back to the Internet as if it had originated from AS200. This is very misleading to ASs connected to the Internet because NetA now has two sources rather than one source, AS100 and AS200.

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.