Information Technology Reference
In-Depth Information
Step 11
With the update from R8, R1 now has three paths, as shown in Table 7-38.
Notice that this is exactly the same BGP RIB as in Step 3. From here, the
same convergence cycle would start over again and continue indefinitely.
Table 7-38
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
Solutions: Setting Proper IGP Metrics Within Confederations
Because path selection does not consider member AS numbers, solutions to this problem
are similar to those provided for route reflection. The following briefly describes each of
the solutions:
•
Use full iBGP mesh
—As with route reflection, this solution might not be acceptable
if confederation is used in the first place to increase iBGP scalability.
•
Enable always-compare-med
—Because MEDs are not modified by member
autonomous systems, the same guidelines and caveats apply as for route reflection.
•
Enable deterministic-MED comparison
—It is almost always a good practice to
enable deterministic MED comparison, although there might be cases in which this
solution alone does not solve the problem.
•
Reset MEDs to 0s
—This eliminates MED as a decision-maker. The same guidelines
apply as for route reflection.
•
Use communities
—Communities exchanged between external neighbors are
primarily private community values, so the same guidelines apply as for route
reflection.
•
Set proper IGP metrics
—If the intra-member-AS metric is set lower than the inter-
member-AS, the intra-member-AS path is preferred over the inter-member-AS paths,
with all other higher-order comparisons being equal. This stops the loops.
Confederation Versus Route Reflection
This chapter introduced two approaches to solve the iBGP scalability issue—confederation
and route reflection. Each method has its strengths and weaknesses, so how do you deter-
mine which method to use during a network design? This section helps you answer that
question by comparing and contrasting the two methods.
