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Redundancy is critical in WAN design for the remote site because of the unreliable nature of WAN links. Most Enterprise Edge solutions require high availability between the primary and remote site. Because WAN links have lower reliability and lack bandwidth, they are good candidates for most WAN backup designs.

Branch offices should have some type of backup strategy in the event of a primary link failure. Backup links can be either dialup or permanent connections.

WAN backup options are as follows:

• Dial backup— ISDN provides backup dialup services in the event of a primary failure of a WAN circuit. The backup link is initiated if a failure occurs with the primary link. The ISDN backup link provides network continuity until the primary link is restored, and then the backup link is terminated such as with floating static route techniques.
• Secondary WAN link— The addition of a secondary WAN link makes the network more fault-tolerant. This solution offers two key advantages:

  - Backup link— Provides for network connectivity if the primary link fails. Dynamic or static routing techniques can be used to provide routing consistency during backup events. Application availability can also be increased because of the additional backup link.

  - Additional bandwidth— Load sharing allows both links to be used at the same time, increasing the available bandwidth. Load balancing can be achieved over the parallel links using automatic routing protocol techniques.



• Shadow PVC— Service providers can offer shadow PVCs, which provide additional PVCs for use if needed. The customer is not charged for the PVC if it does not exceed limits set by the provider while the primary PVC is available. If the limit is exceeded, the service provider charges the customer accordingly.

Load-Balancing Guidelines

Load balancing can be implemented per packet or per destination using fast switching. If WAN links are less than 56 kbps, per-packet load balancing is preferred. Fast switching is enabled on WAN links that are faster than 56 kbps, and per-destination load balancing is preferred.

A major disadvantage of using duplicate WAN links is cost. Duplicate WAN links require additional WAN circuits for each location, and more network interfaces are required to terminate the connections. However, the loss of productivity if a site loses network connectivity and becomes isolated can be greater than the cost of the duplicate WAN link.

WAN Backup over the Internet

Another alternative for WAN backup is to use the Internet as the connectivity transport between sites. However, keep in mind that this type of connection does not support bandwidth guarantees. The enterprise also needs to work closely with the ISP to set up the tunnels and advertise the company's networks internally so that remote offices have reachable IP destinations.

Security is of great importance when you rely on the Internet for network connectivity, so a secure tunnel using IPsec needs to be deployed to protect the data during transport.

Figure 6-3 illustrates connectivity between the headend or central site and a remote site using traditional ATM/FR connections for the primary WAN link. The IPsec tunnel is a backup tunnel that provides redundancy for the site if the primary WAN link fails.

Figure 6-3. WAN Backup over the Internet

IPsec tunnels are configured between the source and destination routers using tunnel interfaces. Packets that are destined for the tunnel have the standard formatted IP header. IP packets that are forwarded across the tunnel need an additional GRE/IPsec header placed on them as well. As soon as the packets have the required headers, they are placed on the tunnel with a destination address of the tunnel endpoint. After the packets cross the tunnel and arrive on the far end, the GRE/IPsec headers are removed. The packets are then forwarded normally using the original IP packet headers