Information Technology Reference
In-Depth Information
The initial BGP RIB on R8 is shown in Table 7-29.
Table 7-29
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 7-30. To
determine the best path, R1 compares Path 1 and Path 2 first. Path 1 wins
because it has a lower IGP metric. Note that the member AS number is
not considered in path selection. So for path selection, Path 1 and Path 2
have the same AS_PATH length. Then Path 1 is compared to Path 3, and
Path 3 wins because it has a lower MED. Again, sub-AS 65001 is not
considered in path selection.
Step 3
Table 7-30
BGP Paths on R1
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1
R4
(65001) 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. For R3, R1 sends a
withdrawal message.
Table 7-31 shows R8's new BGP RIB. Path 1 is the best path because it
has a lower MED. The member AS number is not considered in path
selection. A withdrawal message is sent from R8 to R1.
Step 4
Table 7-31
BGP Paths on R8
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1
*
R3
(65000) 300 400
5
25
2
R4
300 400
6
6
Table 7-32 shows the BGP RIB on R4. Path 1 is best because it has a
lower MED. R4 now withdraws the route sent to R8.
Step 5
Table 7-32
BGP Paths on R4
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1
*
R3
(65000) 300 400
5
31
2
R6
300 400
6
10
