This section describes various backup options for providing alternative paths for remote access. WAN links are relatively unreliable compared to LAN links and often are much slower than the LANs to which they connect. This combination of uncertain reliability, lack of speed, and high importance makes WAN links good candidates for redundancy to achieve high availability. Branch offices should experience minimum downtime in case of primary link failure. A backup connection can be established, either via dialup or by using permanent connections. The main WAN backup options are as follows:

■ Dial backup routing

■ Permanent secondary WAN link

■ Shadow PVC

The following sections describe these options.

Dial Backup Routing

Dial backup routing is a way of using a dialup service for backup purposes. In this scenario, the switched circuit provides the backup service for another type of circuit, such as point-to-point or Frame Relay. The router initiates the dial backup line when it detects a failure on the primary circuit. The dial backup line provides WAN connectivity until the primary circuit is restored, at which time the dial backup connection terminates.

Permanent Secondary WAN Link

Deploying an additional permanent WAN link between each remote office and the central office makes the network more fault-tolerant. This solution offers the following two advantages:

■ Provides a backup link: The backup link is used if a primary link that connects any remote office with the central office fails. Routers automatically route around failed WAN links by using floating static routes and routing protocols, such as EIGRP and OSPF. If one link fails, the router recalculates and sends all traffic through another link, allowing applications to proceed if a WAN link fails, thereby improving application availability.

■ Increased bandwidth: Both the primary and secondary links can be used simultaneously because they are permanent. The routing protocol automatically performs load balancing between two parallel links with equal costs (or unequal costs if EIGRP is used). The resulting increased bandwidth decreases response times.

Shadow PVC

With shadow PVCs, as long as the maximum load on the shadow PVC does not exceed a certain rate (such as one-fourth of the primary speed) while the primary PVC is available, the SP provides a secondary PVC without any additional charge. If the traffic limit on the shadow PVC is exceeded while the primary PVC is up, the SP(Service Provider) charges for the excess load on the shadow PVC.