BGP-filtering

[miracle_david@yahoo.com]

Hi,

In the following link page 15

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.pdf?dtid=osscdc000283

It talks about 129.213.1.0 but access-list permits 175.220.0.0 0.0.255.255 . Why?

[Giuseppe Larosa]

Hello miracle_david@yahoo.com ,

the reason is that the local router owns the prefixes in 175.220.0.0/16.

By using an ACL like that the author is using the implicit deny any at the end of the ACL to filter the prefixes that are not locally generated.

There are other ways to achieve the same result.

The most elegant solution is allowing routes with an empty AS path (the check on the AS path attribute is performed before sending to an eBGP peer that causes the addition of the local AS number in the leftmost position on the AS path so matching on an empty AS path means match locally generated routes in this router and in all routers in the same AS)

Hope to help

Giuseppe

[Abzal]

Hi miracle_david@yahoo.com 

@Giuseppe Larosa gave you perfect explanation.

To avoid such scenarios it's recommended to apply appropriate filters or tags to allow redistribution of only internal routes.