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Using Comparable Inter-AS Metrics in an RR Environment
Using comparable metrics is important for RR designs because RRs reflect only the best
path. Anything that affects an RR's best-path selection that is inconsistent with other peers
in the AS might cause inconsistent and nondeterministic results. The best example of
incomparable metrics is MED. As discussed in Chapter 2, MED is a BGP metric that can
be used to influence inter-AS path selection. MED by default is compared only among the
paths from the same neighboring AS; thus, MEDs from different autonomous systems are
not comparable. Chapter 2 also pointed out that, by default, the order in which paths are
received might affect the outcome of the best-path selection. The following example dem-
onstrates how these two default conditions can potentially lead to a persistent convergence
oscillation in an RR environment.
Problem Description: Incomparable Inter-AS Metrics
Consider the topology shown in Figure 7-17. The prefix 172.16.0.0/16 is advertised by AS
400 to AS 200 and AS 300. When the prefix reaches R2 in AS 100 from R5, the MED is set
to 10. In AS 300, R6 sets a MED of 5 and 6 in updates sent to R3 and R4, respectively,
perhaps to influence the inbound traffic from AS 100 to use the R3-R6 link. Within AS 100,
R1 is an RR with clients of R2, R3, and R4.
Figure 7-17 Persistent Convergence Oscillation
RR
AS 100
R1
Client
Client
R2
R4
Client
R3
MED: 10
MED: 6
MED: 5
AS 200
AS 300
R5
R6
172.16.0.0/16
AS 400
172.16.0.0/16
R7
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