Information Technology Reference
In-Depth Information
The CLUSTER_LIST attribute is created or updated on an RR only during reflection—that
is, when a route is reflected from one client to another client, from one client to a nonclient,
or from a nonclient to a client. If an RR originates a route, the originating RR does not
create the CLUSTER_LIST. When an RR advertises a route to an external peer, the existing
CLUSTER_LIST is removed. When an RR advertises a route learned from an external peer
to a client or nonclient, the RR does not create the CLUSTER_LIST.
NOTE
Using the same configuration as shown in Figure 7-7, Figure 7-9 shows how the CLUSTER_
LIST is used to break a routing information loop in an RR environment. The IP address next
to each router is its RID. When R5 reflects the route to R3 and R4, it creates the CLUSTER_
LIST with its own CLUSTER_ID, 192.168.1.1. By default, the CLUSTER_ID is the router
ID. As R4 reflects the route to R2 and R3, it prepends its own CLUSTER_ID to the list. So
at R3, there are two paths—one with a CLUSTER_LIST of 192.168.1.2, 192.168.1.1, and
the other with a CLUSTER_LIST of 192.168.1.1. By default, R3 prefers the path with the
shortest CLUSTER_LIST (for more information on BGP path selection, refer to Chapter 2),
but because the path via R4 has a higher WEIGHT, the best path for R3 is via R4.
Figure 7-9
How CLUSTER_ID Breaks a Loop
Client
Client
Client
Client
R1
R2
R1
R2
RR/
Client
RR/
Client
RR/
Client
RR/
Client
Weight: 100
Weight: 100
R3
R4
192.168.1.3
192.168.1.2
R3
R4
192.168.1.3
192.168.1.2
RR/
Client
RR/
Client
192.168.1.1
192.168.1.1
X
R5
R5
172.16.0.0/16
172.16.0.0/16
192.168.1.6
Client
192.168.1.6
Client
R6
R6
Initial Updates
R3's Best Path Via R4
When R3 reflects the best path, it prepends its CLUSTER_ID, 192.168.1.3, to the
update. When R5 receives the update, it notices its own CLUSTER_ID in the list, and
the update is denied. Example 7-3 shows what happens on R5, as captured by the
debug ip
bgp update
command.
