Information Technology Reference
In-Depth Information
Because the removed path is not the best path, BGP does not recalculate
the best path yet. So Path 2 is still the best path. When the BGP scanner
runs, the path-selection process is started. Because R2 has a lower RID,
Path 1 is selected as the new best path.
Now R1 needs to update its clients about the new best path. So it sends
updates to R3 and R4. For R2, it sends a withdrawal message. This is
shown in Figure 7-19.
With the new update from R1, R4 has a new BGP RIB, as shown in
Table 7-6. Because Path 2 is learned via an external neighbor (R6), it is
selected as the best path. Next, R4 sends the new path information to R1,
as shown in Figure 7-20.
Step 5
Table 7-6
New Path Information on R4
Path
BGP Next Hop
AS_PATH
MED
1
R2
200 400
10
2
*
R6
300 400
6
Figure 7-20
Prefix Propagation in Steps 5 and 6
R1
R1
RR
RR
NEXT_HOP:
R4
NEXT_HOP:
R3
NEXT_HOP:
R3
Withdraw
R2
R4
R2
R4
Client
Client
Client
Client
R3
R3
Client
Client
Step 5
Step 6
After receiving the new update from R4, R1 now has three paths in its
BGP RIB, as shown in Table 7-7. It steps through the path-selection
process and selects Path 3 as the best path. Now R1 sends updates to its
clients, as shown in Figure 7-20. Note that this is the same RIB as shown
Step 6
