Information Technology Reference
In-Depth Information
Initially, R1 has two paths in its BGP RIB, as shown in Table 7-11. The
IGP metric is the cumulative link metric from the router to the BGP next
hop.
Step 2
Table 7-11
Initial Paths on R1
BGP Next Hop
AS_PATH
MED
IGP Metric
R2
200 400
10
15
R3
300 400
5
20
The initial BGP RIB on R8 is shown in Table 7-12.
Table 7-12
Initial Path on R8
BGP Next Hop
AS_PATH
MED
IGP Metric
R4
300 400
6
6
With the update from R8, R1 has three paths, as shown in Table 7-13. To
determine the best path, R1 first compares Path 1 and Path 2. Path 1 wins
because it has a lower IGP metric. Then Path 1 is compared to Path 3, and
Path 3 wins because it has a lower MED.
Step 3
Table 7-13
BGP Paths on R1
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1
R4
300 400
6
11
2
R2
200 400
10
15
3
*
R3
300 400
5
20
The best path is updated to all R1's neighbors, as shown in Figure 7-22.
For R3, R1 sends a withdrawal message.
