Foundry Networks FESX User Manual

Configuring BGP4
December 2005 © Foundry Networks, Inc. 21 - 35
NOTE: Another method for reducing the complexity of an IBGP mesh is to use route reflection. However, if you
want to run different Interior Gateway Protocols (IGPs) within an AS, configure a confederation. You can run a
separate IGP within each sub-AS.
To configure a confederation, configure groups of BGP routers into sub-ASs. A sub-AS is simply an AS. The term
“sub-AS” distinguishes ASs within a confederation from ASs that are not in a confederation. For the viewpoint of
remote ASs, the confederation ID is the AS ID. Remote ASs do not know that the AS represents multiple sub-ASs
with unique AS IDs.
NOTE: You can use any valid AS numbers for the sub-ASs. If your AS is connected to the Internet, Foundry
recommends that you use numbers from within the private AS range (64512 – 65535). These are private ASs
numbers and BGP4 routers do not propagate these AS numbers to the Internet.
Figure 21.3 shows an example of a BGP4 confederation.
Figure 21.3 Example BGP4 confederation
In this example, four routers are configured into two sub-ASs, each containing two of the routers. The sub-ASs
are members of confederation 10. Routers within a sub-AS must be fully meshed and communicate using IBGP.
In this example, routers A and B use IBGP to communicate. Routers C and D also use IBGP. However, the sub-
ASs communicate with one another using EBGP. For example, router A communicates with router C using EBGP.
The routers in the confederation communicate with other ASs using EBGP.
Routers in other ASs are unaware that routers A – D are configured in a confederation. In fact, when routers in
confederation 10 send traffic to routers in other ASs, the confederation ID is the same as the AS number for the
routers in the confederation. Thus, routers in other ASs see traffic from AS 10 and are unaware that the routers in
AS 10 are subdivided into sub-ASs within a confederation.
Configuring a BGP Confederation
Perform the following configuration tasks on each BGP router within the confederation:
IBGP
Sub-AS 64513
AS 20
This BGP4 router sees all
traffic from Confederation 10
as traffic from AS 10.
Routers outside the confederation
do not know or care that the routers
are subdivided into sub-ASs within a
confederation.
Sub-AS 64512
IBGP
EBGP
EBGP
Confederation 10
Router A Router B
Router C Router D
BGP4 Router

Foundry Networks FESX Specifications

General

Brand	Foundry Networks
Model	FESX
Category	Switch
Language	English