External Border Gateway Protocol (eBGP) 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 4-10, all routers speak eBGP with routers in other autonomous systems. Within AS 500, the routers communicate using iBGP, which is covered next.

Figure 4-10 eBGP Used Between Autonomous Systems

iBGP

Internal Border Gateway Protocol (iBGP) 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 (such as attributes) is forwarded. The various IGPs that might be used do not understand or forward BGP attributes, including autonomous system paths, between eBGP routers.

Another use of iBGP is in large corporations, where the IGP networks are in smaller independent routing domains along organizational or geographic boundaries. In Figure 4-11, a company has decided to use three independent IGPs: one for the Americas; another for Asia and Australia; and another for Europe, the Middle East, and Africa. Routes are redistributed into an iBGP core.

Figure 4-11 iBGP in a Large Corporation

A CCNP enterprise designer should know at a high level these other uses for iBGP:

• Applying policies in the internal autonomous system with the help of BGP path attributes: BGP path attributes are covered later in this chapter.
• QoS policy propagation on BGP (QPPB): QPPB uses iBGP to spread common QoS parameters from one router to other routers in the network. It classifies packets using IP precedence bits based on BGP community lists, BGP autonomous system paths, and access lists. After packets are classified, QoS features can enforce policies.
• Multiprotocol BGP (MP-BGP) peering of Multiprotocol Label Switching (MPLS) virtual private networks (VPNs): The multiprotocol version of BGP is used to carry MPLS VPN information between all provider edge (PE) routers within a VPN community. Multiprotocol Border Gateway Protocol (MP-BGP) is defined in RFC 2858. It introduces a new BGP capabilities advertisement to determine whether a BGP peer supports MP-BGP. It introduces optional nontransitive attributes used to advertise feasible routes to a peer, network layer reachability information, and other characteristics. It defines an address family identifier (AFI) of 2 to identify IPv6, which is used to convey an IPv4 address as the BGP next hop for the advertised IPv6 prefixes.