Enterasys Networks Security Router X-PeditionTM - Manual (Page 166)

Overview
6-18 Configuring the Border Gateway Protocol
Scaling BGP
BGP requires that all BGP speakers with a single AS (IBGP) be fully meshed, as shown in Figure 6-
10 . The result is that for any BGP speakers within an AS, the number of unique BGP sessions
required is determined by the following formula: n x (n-1)/2. Be aware that this fully meshed
requirement does not scale when a large number of IBGP speakers occupy the AS.
Figure 6-10 Fully Meshed BGP
As an example, when an AS contains five routers as shown above, the number of BGP sessions
needed is 10, as set by the equation: 5 x (5-1)/2 = 10.