Information Technology Reference
In-Depth Information
Figure 7-36
Hub-and-Spoke Confederation Design
AS 65001
R5
R6
R7
AS 65000
R9
R11
R1
R2
AS 65002
AS 65003
R8
R12
R3
R4
R10
R13
Confederation
ID 100
R14
R15
AS 65004
R16
Setting Proper IGP Metrics for Confederations
The impact of IGP metrics on path selection in a confederation environment is quite similar
to that in an RR-based environment that deploys multiple clusters, where MEDs are used
to choose paths. In such an environment, IGP metrics should be set so that IGP metrics
within the member AS are lower than those between them, which allows the router to prefer
an intra-sub-AS path over an inter-sub-AS in path selection. Failure to set IGP metrics in
this way might lead to persistent convergence oscillation.
Problem Description: Improper IGP Metrics Within Confederations
Figure 7-37 shows a topology that might lead to convergence oscillation in a confederation.
IGP metrics are given for each link in confederation 100. Other routing information is
similar to that shown in Figure 7-21. The problem description in this section is almost
identical to that presented in the section “Problem Description: Improper IGP Metrics,”
because a member AS is quite similar to an RR cluster in terms of path selection. The
only exception is AS_PATH. With confederation, sub-AS numbers are inserted into the
AS_PATH sent to intraconfederation eBGP peers. However, the best path is not affected,
because sub-AS is not considered in the path selection. This is why the process and the
outcome are the same as in the previous cases involving RR clusters.
