Content Overview Protocols that run inside an enterprise are called interior gateway protocols (IGPs). Examples of IGPs include RIP versions 1 and 2, EIGRP, and OSPF. Protocols that run outside an enterprise, or between autonomous systems, are called exterior gateway protocols (EGPs). Typically, EGPs are used to exchange routing information between Internet Service Providers (ISPs). Since 1994, Border Gateway Protocol version 4 (BGP4) has become the core routing protocol of the Internet. All previous versions are considered obsolete. Most ISPs must use BGP to establish routing between one another. Enterprises typically employ default routes to reach their service providers. However, in some instances, BGP may be more suitable to use between a customer’s autonomous system and the provider's network, such as if an organization has multiple connections to service providers. To help control specific path selections, BGP is an effective alternative to using default routes. Understanding the important characteristics of BGP and the way in which it behaves differently from IGPs is necessary for knowing when and when not to use BGP. A BGP administrator must understand the various options to properly configure BGP for scalable internetworking. This module discusses BGP configuration and verification for enterprise ISP connectivity.
Content 6.1 BGP Concepts and Terminology 6.1.1 Using BGP in the Enterprise Network The Internet is a collection of autonomous systems that are interconnected to allow communication among them. BGP provides the routing between these autonomous systems. Enterprises that want to connect to the Internet do so through one or more ISPs. If an organization has only one connection to one ISP, they probably do not need to use BGP. Instead, they would use a default route. However, if they have multiple connections to one or to multiple ISPs, BGP may be appropriate because it allows them to manipulate path attributes to select the optimal path. To understand BGP, you first need to understand how it differs from the other protocols discussed so far. Routing protocols can be classified as either interior or exterior: BGP is an interdomain routing protocol (IDRP), also known as an EGP. BGP4 is the latest version and is defined in RFC 4271.As noted in this RFC, the classic definition of an autonomous system is “a set of routers under a single technical administration, using an IGP and common metrics to route packets within the autonomous system, and using an inter-autonomous system routing protocol (also called an EGP) to determine how to route packets to other autonomous systems.” Autonomous systems can use more than one IGP, potentially with several sets of metrics. From the BGP point of view, the most important characteristic of an autonomous system is that it appears to other autonomous systems to have a single coherent interior routing plan and presents a consistent picture of reachable destinations. All parts of an autonomous system must connect to each other. When BGP is running between routers in different autonomous systems, it is called External BGP (EBGP). When BGP is running between routers in the same autonomous system, it is called Internal BGP (IBGP). For example, enterprise AS 65500 in Figure is learning routes from both ISP-A and ISP-B via EBGP, and is also running IBGP on all of its routers. AS 65500 learns about routes and chooses the best way to each one based on the configuration of the routers in the autonomous system and the BGP routes passed from the ISPs. If one of the connections to the ISPs goes down, traffic is sent through the other ISP. One of the routes that AS 65500 learns from ISP-A is 172.18.0.0/16. If that route is passed through AS 65500 using IBGP and is mistakenly announced to ISP-B, ISP-B may decide that the best way to get to 172.18.0.0/16 is through AS 65500, instead of through the Internet. AS 65500 would then be considered a transit autonomous system, which is a very undesirable situation. AS 65500 wants to have a redundant Internet connection, but does not want to act as a transit autonomous system between the two ISPs. Careful BGP configuration is required to avoid this situation.
Web Links BGP Support Page
http://www.cisco.com/en/US/tech/tk365/tk80/
tsd_technology_support_sub-protocol_home.html
Content 6.1 BGP Concepts and Terminology 6.1.2 BGP Multihoming Options Multihoming is when an autonomous system has more than one connection to the Internet. Two typical reasons for multihoming are as follows: The benefits of BGP are apparent when an autonomous system has multiple EBGP connections to either single or multiple autonomous systems. Multiple connections allows an organization to have redundant connections to the Internet so that connectivity can still be maintained if a single path becomes unavailable. An organization can be multihomed to either a single ISP or to multiple ISPs. A drawback to having all of your connections to a single ISP is that connectivity issues in that single ISP can cause your autonomous system to lose connectivity to the Internet. By having connections to multiple ISPs, an organization gains the following benefits: A multihomed autonomous system can run EBGP with its external neighbors and might also run IBGP internally. If an organization wants to perform multihoming with BGP, there are three common ways to do this: These options are described in the following topics.
Content 6.1 BGP Concepts and Terminology 6.1.3 Option 1: Default Routes from All Providers Receiving only a default route from each ISP requires the fewest resources within the autonomous system, because a default route is used to reach any external destination. The autonomous system sends all its routes to the ISPs, which process and pass them onto other autonomous systems. If a router in the autonomous system learns about multiple default routes, the local interior routing protocol installs the best default route in the routing table, which is the one with the least-cost IGP metric. This IGP default route routes packets destined to the external networks to an edge router of this autonomous system, which is running EBGP with the ISPs. The edge router uses the BGP default route to reach all external networks. The route that inbound packets take to reach the autonomous system is decided outside the autonomous system (within the ISPs and other autonomous systems). Regional ISPs that have multiple connections to national or international ISPs commonly implement this option. The regional ISPs do not use BGP for path manipulation. However, they require the ability to add new customers and the networks of the customers using BGP. If the regional ISP does not use BGP, each time the regional ISP adds a new set of networks, the