Scalable iBGP Design and Implementation Guidelines - BGP Design and Implementation - page 271

In Figure 7-17, all three border routers—R2, R3, and R4—set next-hop-self when
announcing the prefix to R1. The values of WEIGHT and LOCAL_PREF are at their
defaults. All links have an equal IGP metric of 10.

The neighbor and path information on R1 is shown in Table 7-1. To simplify the discussion,
we will focus on R1 and R4 only.

Table 7-1 Neighbor and Path Information on R1

BGP Next Hop Peer RID AS_PATH MED

R2 192.168.45.4 200 400 10

R3 192.168.45.5 300 400 5

R4 172.16.67.6 300 400 6

The following describes the steps of prefix advertisement that lead to a persistent
convergence loop:

All three border routers receive the prefix from their external neighbor.
The prefix is installed into each router's BGP RIB, with the information
as shown in Table 7-2.

Step 1

Table 7-2 Initial BGP Paths on All Three Border Routers

Router BGP Next Hop AS_PATH MED

R2 R5 200 400 10

R3 R6 300 400 5

R4 R6 300 400 6

All three border routers advertise the prefix via iBGP to R1, as shown in
Figure 7-18 (to simplify the figure, only routers in AS 100 are shown).
The router R1 now has three paths for the prefix, as shown in Table 7-3.
Note that the most recent path is listed at the top and the oldest path is
listed at the bottom, assuming that the path from R3 is received first,
followed by paths from R2 and R4.

Step 2

In the tables in this chapter, the best path is indicated with an asterisk (*). NOTE