Information Technology Reference
In-Depth Information
Solutions to Improper IGP Metrics
Of the several solutions possible, all but one were described in the section “Using
Comparable Inter-AS Metrics in an RR Environment.” The additional solution is to set
proper IGP metrics. Because the result of enabling
deterministic-med
is more complex,
this section focuses on these two solutions.
Enforcing Proper IGP Metric Settings
To set IGP metrics according to rules stated
earlier (intracluster metrics are lower than intercluster metrics), the IGP metric between R1
and R8 is increased from 5 to 50. Now let's walk through the convergence steps again to
see what happens:
Each of the three border routers in AS 100 (refer to Figure 7-21) receives
the prefix from external neighbors and announces the prefix internally
with the next hop set to itself.
Step 1
The prefix is advertised from R2 and R3 to R1 and also from R4 to R8
to R1.
Step 2
The BGP RIB for R1 is given in Table 7-22. Path 1 is compared to Path 2,
and Path 2 is found to be better because it has a lower IGP metric. Path 2 is
then compared to Path 3, and Path 2 is found to be the best again because
of the lower metric. Now R1 updates its neighbors with the new best path.
Step 3
Table 7-22
BGP Paths on R1
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1
R4
300 400
6
56
2
*
R2
200 400
10
15
3
R3
300 400
5
20
Table 7-23 shows the new BGP RIB on R8. Path 2 is the best because it
has a lower IGP metric. Thus, the best path on R8 is unaffected by the
new update from R1.
Step 4
Table 7-23
BGP Paths on R8
Path
BGP Next Hop
AS_PATH
MED
IGP Metric
1
R2
200 400
10
65
2
*
R4
300 400
6
6
