Traditionally, primary and backup WAN links have been deployed using Border Gateway Protocol (BGP) by setting higher preferences to routes learned from a primary link. There are several drawbacks to this solution. Traffic fails over to a backup link only when the primary link goes down or routes are no longer available. When the utilization of the primary link approaches one hundred percent, the router drops packets. Under this condition, the traditional method will choose the primary link for forwarding traffic, and the backup will be unused. Cisco OER can help offload excess traffic from a primary link to a backup link and avoid packet drops.
NAT environment, if the source address changes (different ISP on the backup link), any ongoing TCP connections or User Datagram Protocol (UDP) transactions might fail.
If the source address does not change, the ISP might drop the packet because it will appear not to have originated on the ISPs network.
