Information Technology Reference
In-Depth Information
C H A P T E R
2
Understanding BGP
Building Blocks
The purpose of this chapter is to lay the groundwork for the rest of this topic. No attempt is
made to cover all the basics of BGP, but some of the fundamental BGP components and
concepts are highlighted in this chapter to give you the proper perspective. Wherever appro-
priate, updated information is provided. Specifically, this chapter tries to achieve the fol-
lowing objectives:
Provide an overview of Cisco's implementation of BGP, such as BGP processes in
IOS. A case study on how to estimate BGP memory use in Cisco routers is presented
near the end of this chapter.
Review fundamental BGP components, such as BGP attributes, the BGP decision
process, BGP capabilities exchange, the Routing Information Base (RIB), and so on.
Discuss some of the basic BGP concepts, such as iBGP and BGP and IGP routing
exchange.
Provide an overview of the major switching paths available in Cisco IOS software
and how they relate to the performance of BGP and routers because of resource
contention.
Comparing the Control Plane and Forwarding Plane
A router consists of two logical components: the control plane and the forwarding plane.
The control plane is responsible for building a RIB, which the forwarding plane can use to
classify and forward packets.
A router's performance is closely tied to the performance of both of these planes and how
effectively they coordinate. In a routing architecture design, it is important to understand
the interactions of both planes in regards to packet forwarding and resource contention.
The interaction of the control plane and the forwarding plane and the resulting effect on
BGP performance can be shown in the following example. Processing of BGP protocol
packets involves a lot of computation and data manipulation, especially during conver-
gence. Thus, BGP competes for CPU time with other processes running on the router.
Reducing the number of transit packets (those not directed to the router) being process-
switched (a CPU-intensive operation) by the router can improve BGP performance, espe-
cially during initial convergence. This is because more CPU cycles are available for BGP.
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