Information Technology Reference
In-Depth Information
If all clients are in the same peer group, an RR reflects the prefix received from a client to
all clients, including the client that sources the prefix. Route reflection and peer groups are
discussed later in this chapter.
NOTE
Clustering
Clustering is introduced to provide redundancy in an RR environment. In a traditional
clustering design, multiple RRs are used to serve one or more clients. These RRs are con-
figured with an identical CLUSTER_ID, which is a 4-byte BGP attribute that commonly
takes the form of an IP address and defaults to the BGP router ID. If two routers share the
same CLUSTER_ID, they belong to the same cluster. An advertisement that bears the same
CLUSTER_ID is ignored by the receiving RR in the same cluster. Example 7-1 shows
the output of
debug ip bgp update
as captured on an RR that has the same CLUSTER_ID
(10.0.0.100) as its peer (192.168.12.1).
Updates from an RR That Has the Same CLUSTER_ID Are Denied
Example 7-1
*Jul 3 22:48:51.899: BGP: 192.168.12.1 RR in same cluster. Reflected update
dropped
*Jul 3 22:48:51.899: BGP(0): 192.168.12.1 rcv UPDATE w/ attr: nexthop
192.168.14.4, origin i, localpref 100, metric 0, originator 192.168.24.4,
clusterlist 10.0.0.100, path , community , extended community
*Jul 3 22:48:51.899: BGP(0): 192.168.12.1 rcv UPDATE about 11.0.0.0/8 -- DENIED
due to: reflected from the same cluster;
Over the years, the concept of RR clustering has been expanded to improve redundancy and
design flexibility. Now an RR cluster can include one or more RRs, each with one or more
clients. Figure 7-6 shows two forms of RR clustering. Routers R1, R2, R3, and R4 form one
cluster, as identified by the CLUSTER_ID of 192.168.1.3. Clients R1 and R2 can use either
R3 or R4 to reach other clusters. Note that because R3 and R4 discard the advertisements
sent to each other, R1 and R2 must form iBGP sessions with both R3 and R4.
Figure 7-6 also shows the other form of RR clustering. Both R5 and R6 have R7 as the
client, but R5 and R6 belong to different clusters. Routers R5 and R7 are in the cluster of
192.168.1.1, whereas routers R6 and R7 are in the cluster of 192.168.1.2. It is acceptable
for a client to be in multiple clusters simultaneously. Router R7 can use either R5 or R6 to
reach the other cluster. Prefixes advertised between R5 and R6 are accepted by the receiving
RR because they are not in the same cluster. In this form of clustering, clients must under-
stand RR attributes to prevent potential routing information loops. This can be accom-
plished by having a certain level of IOS release, such as 12.0 or later. Clustering design
is presented later as an example.
