Information Technology Reference
In-Depth Information
The prefix is advertised from R2 and R3 to R1 and also from R4 to R8
to R1.
Step 2
Table 7-25 shows the new BGP RIB on R1. When comparing Path 2 and
Path 3, the best path is Path 2, because it has a lower MED. Path 1 is the
best path because it has a lower IGP metric than Path 2. Next, R1 updates
its neighbors with information on this new best path.
Step 3
Table 7-25 BGP Paths on R1
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1 *
R2
200 400
10
15
2
R3
300 400
5
20
3
R4
300 400
6
11
Table 7-26 shows the BGP RIB on R8. The best path is Path 2 because it
has a lower IGP metric.
Step 4
Table 7-26 BGP Paths on R8
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1
R2
200 400
10
20
2 *
R4
300 400
6
6
No other steps are needed, because the network is now converged. The best path for each
router is the same as shown in Figure 7-26.
Clustering Design
Proper clustering is very important to provide desired redundancy in an RR-based architec-
ture. Consider the topology on the left side of Figure 7-27, in which two RRs use the same
cluster ID. When the prefix 172.16.0.0/16 is advertised from R4, two RRs advertise to R1
and to each other. However, the updates between RRs are discarded because they are in the
same cluster.
Obviously two RRs provide redundancy to clients, but is that enough? The answer depends
on how RRs are configured, as explained in this section. R1 has two BGP paths to the
destination—one learned from R2 and the other learned from R3. Between the two paths,
R1 picks one best path—perhaps the path via R2 (it does not make any difference for this
discussion which path is the best path).
 
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