Scalable iBGP Design and Implementation Guidelines - BGP Design and Implementation - page 273

Prefix propagation is shown in Figure 7-18.

With the new update from R1 and its previous path, R4 now has two paths
to the prefix, as shown in Table 7-4. Path 1 is selected as the best path
because it has a lower MED. Because of this selection, R4 must withdraw
the route sent to R1, as shown in Figure 7-19.

Step 3

Table 7-4 New Path Information on R4

Path BGP Next Hop AS_PATH MED

1 * R3 300 400 5

2 R6 300 400 6

Figure 7-19 Prefix Propagation in Steps 3 and 4

R1

R1

RR RR

Withdraw Withdraw NEXT_HOP:

R2

NEXT_HOP:
R2

R2 R4 R2 R4

Client Client Client Client

R3 R3

Client Client

Step 3

Step 4

After receiving the withdrawal message, R1 removes the path from R4.
Now R1 has only two paths left, as shown in Table 7-5.

Step 4

Table 7-5 New Paths on R1

Path BGP Next Hop AS_PATH MED RID

1 R2 200 400 10 192.168.45.4

2 * R3 300 400 5 192.168.45.5