customers must wait until the national ISPs add these networks to their BGP process and place static routes pointing at the regional ISP. By running EBGP with the national or international ISPs, the regional ISP needs to add only the new networks of the customers to its BGP process. These new networks automatically propagate across the Internet with minimal delay. A customer that chooses to receive default routes from all providers must understand the limitations of this option: In Figure , AS 65000 and AS 65250 send default routes into AS 65500. The IGP metric that is used to reach the default route within the autonomous system determines which ISP a specific router within AS 65500 uses. For example, if you use RIP within AS 65500, router C selects the route with the lowest hop count to the default route when sending packets to network 172.16.0.0.
Content 6.1 BGP Concepts and Terminology 6.1.4 Option 2: Default Routes and Partial Updates In this multihoming design option, all ISPs pass default routes and select specific routes to the autonomous system. An enterprise running EBGP with an ISP that wants a partial routing table generally receives the networks that the ISP and its other customers own. The enterprise can also receive the routes from any other autonomous system. Major ISPs are assigned between 2000 and 10,000 classless interdomain routing (CIDR) blocks of IP addresses from the Internet Assigned Numbers Authority (IANA), which they reassign to their customers. If the ISP passes this information to a customer that wants only a partial BGP routing table, the customer can redistribute these routes into its IGP. The internal routers of the customer (these routers are not running BGP) can then receive these routes via redistribution. They can take the nearest exit point based on the best metric of specific networks, instead of taking the nearest exit point based on the default route. Acquiring a partial BGP table from each provider is beneficial because path selection is more predictable than when using a default route. In Figure , ISPs in AS 65000 and AS 64900 send default routes and the routes that each ISP owns to AS 64500. The enterprise (AS 64500) asked both providers to also send routes to networks in AS 64520 because of the amount of traffic between AS 64520 and AS 64500. By running IBGP between the internal routers within AS 64500, AS 64500 can choose the optimal path to reach the customer networks (AS 64520, in this case). The routes to AS 64100 and to other autonomous systems that are not specifically advertised to AS 64500 by ISP A and ISP B are decided by the IGP metric that is used to reach the default route within the autonomous system.
Content 6.1 BGP Concepts and Terminology 6.1.5 Option 3: Full Routes from All Providers In the third multihoming option, all ISPs pass all routes to the autonomous system, and IBGP is run on all the routers in the transit path in this autonomous system. This option allows the internal routers of the autonomous system to take the path through the best ISP for each route. This configuration requires a lot of resources within the autonomous system, because it must process all of the external routes. The autonomous system sends all of its routes to the ISPs, which process the routes and pass them to other autonomous systems. In Figure , AS 65000 and AS 64900 send all routes into AS 64500. The ISP that a specific router within AS 64500 uses to reach the external networks is determined by the BGP protocol. The routers in AS 64500 can be configured to influence the path to certain networks. For example, routers A and B can influence the outbound traffic from AS 64500.
Content 6.1 BGP Concepts and Terminology 6.1.6 BGP Routing Between Autonomous Systems The main goal of BGP is to provide an interdomain routing system that guarantees loop-free exchange of routing information between autonomous systems. Routers exchange information about paths to destination networks. BGP is a successor of the Exterior Gateway Protocol, which was developed to isolate networks from each other as the Internet grew. It is important not to confuse the Exterior Gateway Protocol with the category of EGP. There are many RFCs relating to BGP4, the current version of BGP, including 1772, 1773, 1774, 1930, 1966, 1997, 1998, 2042, 2385, 2439, 2545, 2547, 2796, 2858, 2918, 3065, 3107, 3392, 4223, and 4271. BGP4 has many enhancements over earlier protocols. The Internet uses BGP4 exclusively to connect enterprises to ISPs and to connect ISPs to each other. BGP4 and its extensions are the only acceptable versions of BGP available for use on the public-based Internet. BGP4 carries a network mask for each advertised network and supports both variable-length subnet masking (VLSM) and CIDR. BGP4 predecessors did not support these capabilities, which are currently mandatory on the Internet. When CIDR is used on a core router for a major ISP, the IP routing table, which is composed mostly of BGP routes, has more than 170,000 CIDR blocks. Not using CIDR at the Internet level would cause the IP routing table to have more than 2,000,000 entries. Using BGP4 and CIDR prevents the Internet routing table from becoming too large for interconnecting millions of users. Autonomous System Numbers
Recall that an autonomous system is a collection of networks under a single technical administration. IGPs operate within an autonomous system, and BGP (specifically BGP4) is used between autonomous systems on the Internet. IANA is the organization that maintains records of global IP address allocation. The Regional Internet Registry (RIR) is an organization overseeing the allocation and registration of Internet number resources within a particular region of the world. Resources include IP addresses (both IPv4 and IPv6) and autonomous system numbers. There are currently five RIRs. The American Registry for Internet Numbers (ARIN) has the jurisdiction to assign numbers for the Americas and some islands in the Caribbean. Réseaux IP Européens Network Coordination Center (RIPE NCC) administers autonomous system numbers for Europe, the Middle East, and Central Asia. The Asia Pacific Network Information Center (APNIC) administers the numbers for the Asia-Pacific region. Latin American and Caribbean Internet Addresses Registry (LACNIC) is responsible for Latin America and some of the Caribbean. AfriNIC is responsible for Africa. Autonomous system numbers are 16-bits, ranging from 1 to 65535. RFC 1930 provides guidelines for the use of numbers. The numbers 64512 through 65535 are reserved for private use, much like private IP addresses. The autonomous system numbers used in this course are all in the private range to avoid publishing numbers belonging to organizations. Note
Using an IANA-assigned autonomous system number rather than a private number is necessary only if your organization plans to use an EGP, such as BGP, and connect to a public network, such as the Internet. Comparison with IGPs
BGP works differently than IGPs. An internal routing protocol looks for the quickest path from one point in a corporate network to another based on certain metrics. RIP uses hop counts that look to cross the fewest Layer 3 devices to reach the destination network. OSPF and EIGRP look for the best speed according to the bandwidth statement on the interface. All internal routing protocols look at the path cost to a destination. In contrast, BGP, an external routing protocol, does not look at speed for the best path. Rather, BGP is a policy-based routing (PBR) protocol that allows an