Information Technology Reference
In-Depth Information
When the prefix 172.16.3.0/24 is advertised from R2 to R3, a community of
no-advertise
is attached to it. When R3 receives the prefix, it does not advertise the prefix further to R4.
Thus, the prefix is not known on R4 and R5.
Confederation External and Confederation Internal Routes
A
confederation external route
is a route received from a confederation external peer,
whereas a
confederation internal route
is received from an internal peer. From a path-
selection perspective, there is no distinction between the two.
Private AS Numbers
The member AS numbers used within the confederation are never visible from outside the
confederation. Thus, a private AS number (ranging from 64512 through 65535) is typically,
but not necessarily, used to identify the member AS inside a confederation without the need
to coordinate AS number assignment with an official AS delegation authority. In IOS, all
member AS numbers are removed automatically at confederation borders, so no manual
configuration is needed to remove the member AS number.
Confederation Design Examples
When designing a confederation architecture, you should follow certain guidelines to
reduce complexity and routing issues. This section presents the following two design
examples:
•
Hub-and-spoke architecture
•
Setting the proper IGP metrics for confederations
Hub-and-Spoke Architecture
A
hub-and-spoke confederation architecture
is one that has one member AS as the
backbone and that also acts as a transit sub-AS that connects to all the other member
autonomous systems. Figure 7-36 shows an example of this architecture, in which AS
65000 is the backbone transit AS for the confederation 100.
The benefits of this architecture are as follows:
•
None of the other subautonomous systems have to be connected directly to each other,
which reduces the number of intraconfederation eBGP sessions.
•
The hub-and-spoke confederation results in a predictable and consistent AS_PATH
within the confederation. For traffic from one nontransit member AS to any other
nontransit member, autonomous systems always take two AS hops. For example,
traffic from AS 65002 takes two hops to AS 65001, 65003, or 65004.
