Correct Answer: A
BGP is usually configured between two directly connected routers that belong to different autonomous systems. Each autonomous system is under different
technical administration. BGP is frequently used to connect the enterprise to service providers and to interconnect service providers. The routing protocol within
the enterprise could be any Interior Gateway Protocol (IGP). Common IGP choices include RIPv2, EIGRP, OSPF, IS-IS. BGPv4 is the only deployed Exterior
Gateway Protocol (EGP). BGP is an interdomain routing protocol that allows BGP speakers residing in different autonomous systems to exchange routing (NLRI)
information. An autonomous system is a collection of devices under common administration. BGP autonomous systems range from 1 through 65,535.
Autonomous system numbers (ASN) 1 through 64, 511 are considered public ASNs. These are allocated by IANA to Regional Internet Registries (RIR). Entities
wanting to receive an ASN must complete the application process of their local RIR and be approved before being assigned an ASN. ASNs 65,512 through 65,535
are considered private ASNs. These ASNs can be used by any organization, but, like RFC 1918 addresses, cannot be used on the Internet. Before two BGP
routers can exchange routing updates, they must become established neighbors
After BGP routers establish a TCP connection, exchange information, and accept the information, they become established neighbors and start exchanging
routing updates. If the neighbors do not reach an established state, they do not exchange BGP updates. The information exchanged before the neighbors are
established includes the BGP version number, ASN, BGP router ID, and BGP capabilities.
External Border Gateway Protocol is the term used to describe BGP peering between neighbors in different autonomous systems. As required by RFC
1771, the eBGP peers share a common subnet (although Cisco does allow some flexibility to avoid doing so). In Figure 11-9, all routers speak eBGP with routers
in other autonomous systems. Within autonomous system 500, the routers communicate using iBGP, which is covered next.
Internal Border Gateway Protocol is the term used to describe the peering between BGP neighbors in the same autonomous system. iBGP is used
primarily in transit autonomous systems. Transit autonomous systems forward traffic from one external autonomous system to another external autonomous
system. If transit autonomous systems did not use iBGP, the eBGP-learned routes would have to be redistributed into an IGP and then redistributed into the BGP
process in another eBGP router. Normally, the number of eBGP routes is too large for an IGP to handle. iBGP provides a better way to control the routes within
the transit autonomous system. With iBGP, the external route information (attributes) is forwarded. The various IGPs that might be used do not understand or
forward BGP attributes, including autonomous system paths, between eBGP routers.
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