Redistributing and Filtering with BGP

An enterprise border router running BGP typically announces only the major network (the prefix assigned to the enterprise network) to the external domains, excluding any details about subnets. This is done using the BGP network router configuration command, which allows BGP to advertise a network that is already part of its IP routing table.

Alternatively, internal networks could be summarized into one major subnet that covers the assigned public address space and redistributed into BGP. However, redistributing from an IGP into BGP is not recommended, because any change in the IGP routes—for example, if a link goes down—can cause a BGP update, which might result in unstable BGP tables.

If IGP routes are redistributed into BGP, make sure that only local routes—those that originate within the AS—are redistributed. For example, routes learned from other autonomous systems (that were learned by redistributing BGP into the IGP) must not be sent out from the IGP again, because routing loops could result, or the AS could inadvertently become a transit AS. Private IP addresses must not be redistributed, so they should also be filtered. Configuring this filtering can be complex.

In the other direction, either a default route or a default route plus a few other specific routes is passed into an enterprise AS. These can then be redistributed into the IGP running in the AS.

Redistributing all BGP routes into an IGP is not advised, because non-BGP participating routers do not require full Internet routing tables, and IGP protocols are unable to process large numbers of advertised routes. Unnecessary routes should be filtered.

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