Although the alternatives to BGP-4 are preferred for their simple connections to the Internet, on some occasions it is advantageous to use the power of this complex protocol. These reasons to use BGP-4 include the following:
• Your organization is connecting to multiple ISPs or autonomous systems and is actively using those links. Many organizations use this for redundancy purposes, justifying the additional cost by using all the links and reducing bottlenecks and congestion. In this case, policy routing decisions may need to be made on a link-by-link basis.
• The routing policy of the ISP and your company differ. The cost of the link depends on usage; time of day and other factors may need to be programmed into the BGP-4 configuration to make the best use of the connection by manipulating the traffic.
• The traffic in your organization needs to be distinguished from that of the ISP. They cannot logically appear as one autonomous system.
• Your organization is an ISP and, therefore, conforms to all of the previous criteria. The nature of your business requires the traffic from other autonomous systems to travel across your autonomous system, treating it as a transit domain.
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