Information Technology Reference
In-Depth Information
BGP Configuration to Reset MED to 0 on R4
Example 7-4
neighbor 192.168.46.6 remote-as 300
neighbor 192.168.46.6 route-map Med-reset in
!
route-map Med-reset permit 10
set metric 0
Using Communities
When communities are used to set BGP policies, MEDs are reset
on the border routers, and the routing policy can be derived from the inbound community
values. This effectively removes MED's impact. Because community is not evaluated in the
path selection, a scheme of community values versus routing preference settings should
be created and communicated to administrators of the neighboring autonomous systems.
Chapter 9, “Service Provider Architecture,” discusses how to design a coherent routing
policy based on BGP communities.
Setting Proper IGP Metrics in an RR Environment
In the previous example, you learned that inconsistent inter-AS metric (MED) can cause
persistent convergence loops. In this section, you will see that improper IGP metrics can
also lead to persistent convergence loops in an RR environment.
In iBGP best-path selection, IGP metrics are often one of the tiebreakers. In a multicluster
RR architecture, IGP metrics should be set in such a way that intracluster metrics are lower
than intercluster metrics, which allows an RR to select an intracluster path over an inter-
cluster path. Failure to set IGP metrics properly might lead to persistent convergence
oscillation.
Problem Description: Improper IGP Metrics
Figure 7-21 shows a topology that potentially leads to an infinite convergence loop. This
topology is similar to that shown in Figure 7-17, except that the IGP metrics are different
and AS 100 has two RR clusters.
