Scalable iBGP Design and Implementation Guidelines - BGP Design and Implementation - page 280

Figure 7-22 Prefix Propagation in Steps 3 and 4

R8 R1 R8

R1

RR RR

RR RR

NEXT_HOP:
R3

Withdraw

NEXT_HOP:
R3

NEXT_HOP:
R3

Withdraw

R2 R3 R4 R2 R3 R4

Client Client Client Client Client Client

Step 3 Step 4

Table 7-14 shows the new BGP RIB of R8. Path 1 is the best path because
it has a lower MED. Figure 7-22 shows the updates sent to neighbors
from R8. Because the path via R1 becomes the best path, R8 sends a
withdrawal message to R1.

Step 4

Table 7-14 BGP Paths on R8

Path BGP Next Hop AS_PATH MED IGP Metric

1 * R3 300 400 5 25

2 R4 300 400 6 6

Table 7-15 shows the BGP RIB on R4. Path 1 is best because it has a
lower MED. This causes R4 to send a withdrawal message to R8, as
shown in Figure 7-23.

Step 5

Table 7-15 BGP Paths on R4

Path BGP Next Hop AS_PATH MED IGP Metric

1 * R3 300 400 5 31

2 R6 300 400 6 10