Chapter 7 BGP
BGP is an example of an EGP and is very robust and scalable routing protocol; BGP is the routing
protocol used on the Internet.
The main goal of BGP is to provide an interdomain routing system that guarantees the loop-free
exchange of routing information between autonomous system. BGP routers exchange information
about path to destinations networks.
BGP´s interdomain routing enables connectivity between autonomous system and is usually based on a
set of policies, not just the technical characteristics of the underlying infrastructure. This capability
differentiates BGP from the IGPs, which focus only on finding the optimum (usually fastest) route
between points, without respect to routing policies.
BGP does not look at bandwidth for the best path. Rather, BGP is a policy-based routing protocol that
allows an autonomous system to control traffic using multiple BGP attributes. Routers running BGP
exchange network reachability information, called path vectors or attributes, including a list of the full
path of BGP autonomous system numbers that a router should take to reach a destination network.
BGP allows some organizations to fully use all of its bandwidth by manipulating these path attributes.
BGP is designed to scale to huge internetworks, such as the internet.
BGP allows routing-policy decisions to be applied to the path of BGP autonomous system numbers so
that routing behavior can be enforced at the autonomous system level and to determine how data will
flow through the autonomous system. These policies can be implemented for all networks owned by an
autonomous system, for a certain classless interdomain routing (CIDR) block of network numbers
(prefixes), or for individual networks or subnetworks. The policies are based on the attributes carried in
the routing information and configured on the routers.
BGP specifies that a BGP router can advertise to its peers (neighbors) in neighboring autonomous
system only those routes that it uses. This rule reflects the hop-by-hop routing paradigm generally used
throughout the current Internet. Some policies cannot be supported by the hop-by-hop routing
paradigm. For example, BGP does not allow one autonomous system to send traffic to a neighboring
autonomous system, intending that the traffic take a different route from that taken by traffic
originating in that neighboring autonomous system. In other words, you cannot influence how a
neighboring autonomous system will route your traffic, but you can influence how your traffic gets to a
neighboring autonomous system. However, BGP can support any policy conforming to the hop-by-hop
routing paradigm.
Because the current Internet uses only the hop-by-hop routing paradigm, and because BGP can support
any policy that conforms to that paradigm, BGP is highly applicable as an inter-autonomous system
routing protocol for the current Internet
