Information Technology Reference
In-Depth Information
The top-level RRs, Level 2 RRs in Figure 7-10, must be fully meshed, because they are not
clients of any RRs. The number of iBGP sessions in this example is 22, compared to 66 in
a full mesh.
Rules for prefix advertisement for the hierarchical RRs are the same as for single-level RRs.
Figure 7-11 shows an example. The prefix 172.16.0.0/16 is received by two border routers
in AS 100. To simplify the discussion, the following focuses on one of the routers, R7.
Figure 7-11
Prefix Advertisement Using Hierarchical Route Reflection
Level 2 RRs
AS
200
R13
R2
R1
R14
R3
R4
Level 1 RRs/
Clients
R8
R5
R6
R7
R10
R9
R11
R12
Clients
Clients
AS
100
As a Level 1 RR, R7 advertises the prefix to its clients, R11 and R12. At the same time, R7
advertises the prefix to its nonclients, R2 and R4. Routers R2 and R4 reflect the prefix to
each other, to the other client, and to other Level 2 RRs. Note that R1 and R3 do not
advertise the prefix to each other, because they are regular iBGP peers with respect to
the neighbors that advertised the prefix.
As RRs, R1 and R3 further propagate the prefix to their clients, R5 and R6. In turn, R5 and
R6 advertise the prefix to their clients, R9 and R10. Now the entire domain is populated
with the prefix.
It is important to remember that RRs reflect only the best path—not the entire path infor-
mation. Even though hierarchical RRs behave the same as single-level RRs in this respect,