Design Reference

In the preceding figure, consider the following:
• The AS is divided into three regions that each run different and independent IGPs.
• Regions logically interconnect by using a full-mesh IBGP, which also provides Internet
connectivity.
• Internal non-BGP routers in each region default to the BGP border router, which contains all
routes.
• If the destination belongs to another region, the traffic is directed to that region; otherwise, the
traffic is sent to the Internet connections according to BGP policies.
To configure multiple policies between regions, represent each region as a separate AS. Implement
EBGP between ASs, and implement IBGP within each AS. In such instances, each AS injects its
IGP routes into BGP, where they are propagated to all other regions and the Internet.
The following figure shows the use of EBGP to join several ASs.
Figure 29: Multiple regions separated by EBGP
You can obtain AS numbers from the Inter-Network Information Center (NIC) or use private AS
numbers. If you use private AS numbers, be sure to design your Internet connectivity carefully. For
example, you can introduce a central, well-known AS to provide interconnections between all private
ASs and the Internet. Before it propagates the BGP updates, this central AS strips the private AS
numbers to prevent them from leaking to providers.
The following figure illustrates a design scenario in which you use multiple OSPF regions to enable
peering with the Internet.
Border Gateway Protocol
December 2014 Network Design Reference for Avaya VSP 4000 Series 69
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