CCNP Routing Study Guide- P13:T his book is intended to help you continue on your exciting new path toward obtaining your CCNP and CCIE certification. Before reading this book, it is important to have at least read the Sybex CCNA: Cisco Certified Network Associate Study Guide, Second Edition. You can take the CCNP tests in any order, but you should have passed the CCNA exam before pursuing your CCNP. | 324 Chapter 9 BGP Scalability and Advanced Features neighbor filter-list 1 out neighbor remote-as 300 neighbor filter-list 11 in neighbor filter-list 1 out ip as-path access-list 1 permit A200 ip as-path access-list 10 permit A100 ip as-path access-list 11 permit A300 To implement filters use the neighbor command. Using the AS path syntax you can configure filters to block routes that contain the AS path information that does not match the regular expression. The output above shows access list 1 allowing only routes that originate from AS 200 to be sent to the respective neighbors. Access lists 10 and 11 above allow only routes that do not originate within AS 100 and AS 300 to be sent. Creating BGP Policies e use policies with BGP to tell other BGP neighbors the paths through our own network. By not advertising certain routes through our network we keep other networks from learning about them it is difficult to route a packet through a network you don t know about. We can modify routes that we wish to advertise using both prefix lists and distribute lists. Distribute lists use access lists to control the routes advertised by a routing protocol. A prefix list is similar to an access list but is more flexible and less complicated to configure than an access list. Distribute Lists Distribute lists are standard or extended access lists applied to a router s BGP session to permit or deny advertised routes through the network. Distribute lists can be applied to filter BGP advertisements either coming in or going out Copyright 2001 SYBEX Inc. Alameda CA Creating BGP Policies 325 of the router. Let s look at an example of an access list that allows routes from network . RouterA config access-list 105 permit ip host There is always an implicit deny all at the end of the access list that can t be seen. We re permitting only network in this access list. .
