Information Technology Reference
In-Depth Information
C
H
A
P
T
E
R
7
Scalable iBGP Design and
Implementation Guidelines
In a typical ISP environment, an AS contains many BGP-speaking routers. Fully meshing
all these iBGP speakers would result in both a high BGP session count and high resource
consumption per router. This chapter focuses on two common solutions—route reflection
and confederation. Using these two solutions, this chapter demonstrates practical guide-
lines with extensive examples of how to design a scalable iBGP routing architecture.
Through four case studies in Chapter 8, “Route Reflection and Confederation Migration
Strategies,” which contain step-by-step migration procedures, you will further explore the
subject of iBGP scalability.
Issues of iBGP Scalability
As you recall, the subject of loop prevention mechanisms in BGP was introduced in Chap-
ter 2, “Understanding BGP Building Blocks.” When BGP is used to distribute reachability
information among a series of autonomous systems, as is the case with eBGP, the BGP
attribute AS_PATH is used as the loop-prevention mechanism. An eBGP speaker discards
any BGP updates it receives from an eBGP peer that contains its own AS number. Because
the AS_PATH attribute is preserved within the same AS, a different loop-prevention mech-
anism must be employed for iBGP. The rule is simply that an iBGP speaker does not relay
or readvertise reachability information received via iBGP from one iBGP speaker to anoth-
er iBGP speaker. For example, if routers R1, R2, and R3 are all iBGP-only speakers within
the same AS, and R2 receives a prefix from R1, R2 does not send that prefix via iBGP to R3.
The loop-prevention mechanism within iBGP forces all iBGP speakers to have BGP
sessions with each other. In other words, they are fully meshed so that all BGP speakers can
receive full routing information. In the example given in the preceding paragraph, fully
meshed means that R1 needs to have BGP sessions with R2 and R3. Also, R2 and R3 must
peer with each other via iBGP. The total number of iBGP sessions among n iBGP routers
is
n
(
n
-1)/2, with each router having (
n
-1) sessions. Figure 7-1 shows the relationship
between the total number of iBGP sessions and the number of fully meshed iBGP routers,
commonly called the
n
2
relationship. For example, when the number of iBGP routers
increases from 10 to 100, the total number of iBGP sessions increases from 45 to 4950!
