WAN Backup Strategies

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

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.

NOTE A floating static route is one that appears in the routing table only when the primary route goes away. The administrative distance of the static route is configured to be higher than the administrative distance of the primary route, and it "floats" above the primary route until the primary route is no longer available.

■ 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.

Cost is the primary disadvantage of duplicating WAN links to each remote office. For example, in addition to new equipment, including new WAN router interfaces, a large star network with 20 remote sites might need 20 new virtual circuits.

In Figure 5-17, the connections between the Enterprise Edge and remote sites use permanent primary and secondary WAN links for redundancy. A routing protocol, such as EIGRP, that supports load balancing over unequal paths on either a per-packet or per-destination basis is used to increase the utilization of the backup link.

Figure 5-17 Permanent Secondary WAN Link

If the WAN connections are relatively slow (less than 56 kbps), per-packet load balancing should be used. Load balancing occurs on a per-destination basis when fast switching is enabled, which is appropriate on WAN connections faster than 56 kbps.

Switching Modes: Process, Fast, and Other Modes

During process switching, the router examines the incoming packet and looks up the Layer 3 address in the routing table, which is located in main memory, to associate this address with a destination network or subnet. Process switching is a scheduled process performed by the system processor. Compared to other switching modes, process switching is slow because of the latency caused by scheduling and the latency within the process itself.

With fast switching, an incoming packet matches an entry in the fast-switching cache (also called the route cache), which is located in main memory. This cache is populated when the first packet to the destination is process-switched. Fast switching is done via asynchronous interrupts, which are handled in real time and result in higher throughput.

Other switching modes are available on some routers (including Autonomous Switching, Silicon Switching, Optimum Switching, Distributed Switching, and NetFlow Switching). Cisco Express Forwarding (CEF) technology, described in Chapter 4, is the latest advance in Cisco IOS switching capabilities for IP.

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 charges for the excess load on the shadow PVC.

Figure 5-18 illustrates redundant connections between remotes sites and the Enterprise Edge using the shadow PVCs offered by the SP. Because of the potential for additional costs, the routers must avoid sending any unnecessary data (except, for example, routing traffic) over the shadow PVC.

Figure 5-18 Shadow PVC

Figure 5-18 illustrates redundant connections between remotes sites and the Enterprise Edge using the shadow PVCs offered by the SP. Because of the potential for additional costs, the routers must avoid sending any unnecessary data (except, for example, routing traffic) over the shadow PVC.

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