BROCADE IP PRIMER

Chapter 13: Border Gateway Protocol (BGP)In the image, eBGP is used from each router to communicate to their respectiveISP. Between your routers, iBGP is used to share the information obtainedfrom each ISP.OSPF, RIP or any other IGP could be used to share the routes between Router Aand Router B, but there's a cost. Let's say you wanted to use OSPF. OSPF is alink state protocol. It uses cost to determine the best route. BGP (whether interioror exterior) uses path to determine the best route. Any time you convertfrom one protocol to another, you may not get the most accurate portrayal ofwhich route is the best. Just as in translating languages from one to another,sometimes the meaning gets “lost in the translation.” With iBGP, on the otherhand, you're using an IGP, but an IGP that uses the same metrics (path vector),so nothing is lost in translation between eBGP and iBGP. Nothing needs to betranslated.iBGP may also be used without necessarily tying in with eBGP. It is an IGP. Itcould be used in place of RIP, OSPF, or any other IGP. Commonly, iBGP will bedeployed in either a large scale environment (often in connecting two companieswho may have merged) or, as we showed earlier, in multihoming.The Autonomous System (AS)With BGP, the Internet is broken up into smaller subgroups called AutonomousSystems (AS). Autonomous systems, when pertaining to BGP, refer to a collectionof routers that are all under the same administrative entity. It's easy to getthese confused with the autonomous systems we described in OSPF. They'resimilar, but not always the same. In OSPF, the autonomous system was a collectionof routers running the same routing protocol. In BGP, the autonomoussystem is still a collection of routers, but they could be running multiple IGPsamongst themselves (and still be considered a single autonomous system).The key with BGP is that the collection of routers is administrated (or maintained)by a single entity.For example, consider a business with offices in Chicago, Salt Lake City, andLos Angeles. Should this business interface with the Internet, commonly itwould do so as a single AS. It has offices that are geographically diverse, butall of the routers in the networks are maintained by the same entity.In BGP, an autonomous system is designated by a number: the AS number.This number is 16 bits, so it can be any number from 1 to 65,535. However,just like IP addresses that are used on the Internet, AS numbers must be publiclyregistered. The good news is, the place to publicly register IP addressesand the place to publicly register AS numbers are the same.274 Brocade IP Primer