In some circumstances, multiple paths to an ISP are important to have. There are business and technical reasons to configure a BGP network in a multihomed configuration. Mission-critical applications often call for redundant network designs. When access to applications is provided over the Internet, enterprises typically use multihomed BGP networks to achieve their goals of high availability. Full BGP routing tables and default BGP route selection might ordinarily be considered desirable characteristics of a network. However, the overhead of full BGP routing tables is not warranted in these situations. Furthermore, the default route selection in BGP often does not match the business and technical requirements for multihomed enterprise networks that use BGP.

In network implementations that require connections to multiple ISPs, you will typically use AS path filters to influence BGP route selection. As a network administrator, you must understand the syntax of an AS path regular expression. You also must understand how string-matching operators function when they are using AS path regular expressions to match BGP routes. BGP allows connectivity between multiple ISPs for redundancy and scalability. You can employ AS path filters to remedy the problems that are associated with the various connectivity methods that are used within BGP. This course explains the methods that are used to implement BGP AS path filters.

Where multiple paths between a customer and an ISP exist, certain information must be filtered during BGP updates to influence route selection or to enforce an administrative policy. To meet this requirement, you must use filters. The use of prefix lists is typically easier than the use of standard IP access lists, and it provides performance benefits. You must understand the commands to apply filtering of inbound or outbound updates with prefix lists and where they should be applied. In this course, you will learn the requirements for using prefix-based filters in customer implementations where connections to multiple ISPs must be supported. You will also learn the advantages of prefix lists over IP access lists. This course discusses the commands used to apply filtering of inbound or outbound updates with prefix lists and to configure prefix list filters, and also where network administrators should apply them.

You can use ORF as an extra mechanism to minimize the number of updates that are requested from a neighbor. It will reduce the link bandwidth consumption and CPU use when a router requests a route refresh. An ORF also allows filtering of information that external networks should not receive (such as RFC 1918 information). Understanding how to monitor outbound route filtering capabilities is also important. A BGP neighbor that supports specific ORF capabilities will report those capabilities to a monitoring neighbor and can then send a filter of the supported type to the neighbor. This course discusses the function of outbound route filtering in a BGP network. It also discusses the format and function of ORF messages and the commands that enable ORF negotiations and the activation of an ORF prefix list. It also details the commands that are used to trigger a route refresh and discusses how to monitor the operations of a configured ORF in a BGP network.

BGP is a powerful routing protocol that supports a wide variety of administrative policy controls and route selection features. You cannot achieve many complex filtering goals and administrative policies by using only single-purpose filtering methods or by compounding multiple filtering methods together. Route maps provide a method to perform various compound, complex filtering operations within a single tool. You must understand the operation and use of route maps to successfully implement any large-scale BGP deployment. In this course, you will learn about route maps and how you can use them for BGP filtering. You will learn the commands that are required to use route maps with prefix lists. You will also learn how to use prefix lists as BGP filters and how to monitor previously configured route maps.

Because BGP can handle large volumes of routing information, traditional routing update methods are not feasible. Routing policies for a BGP neighbor may include filtering mechanisms such as route maps, distribute lists, prefix lists, and AS path filter lists. Each of these filters may affect inbound or outbound routing table updates. Whenever there is an administrative change in a routing policy, the BGP session must be reset before the new policy can take effect. To accomplish this task, you can use two types of reset: hard reset and soft reset. Clearing a BGP session using a hard reset invalidates the cache and results in a negative impact on the operation of networks because the information in the cache becomes unavailable. A soft reset allows routing tables to be reconfigured and activated without clearing the BGP session and thus is recommended. This course discusses routing updates in a BGP environment and the traditional methods of forcing BGP route updates after changes in a filter policy. It also discusses the function and benefits of soft reconfiguration and route refresh. The course also presents the commands that are required to perform a soft reconfiguration and route refresh, and explains how to monitor and troubleshoot these features.